Refactor Sem multi-group support

[alyst]

This is a largest remaining part of #193, which changes some interfaces.

Refactoring of the SEM types

• AbstractLoss is the base type for all functions
• SemLoss{O,I} <: AbstractLoss is the base type for all SEM losses, it now requires to have observed::O and implied::I field
• Since SemLoss ctor should always be given observed and implied (positional), meanstructure keyword is gone -- loss should always respect implied specification.
• LossTerm is a thin wrapper around AbstractLoss that adds optional id of the loss term and optional weight
• Sem is a container of LossTerm objects (accessible via loss_terms(sem), or loss_term(sem, id)), so it can handle multiple SEM terms (accessible via sem_terms(sem) -- subset of loss_terms(sem), or sem_term(sem, id)).
  It replaces both the old Sem and SemEnsemble.
• AbstractSingleSem, AbstractSemCollection and SemEnsemble are gone.

Method changes

Multi-term SEMs could be created like

model = Sem(
    :Pasteur => SemML(obs_g1, RAMSymbolic(specification_g1)),
    :Grant_White => SemML(obs_g2, RAM(specification_g2)),
    ...
)

Or with weights specification

model = Sem(
    :Pasteur => SemML(obs_g1, RAMSymbolic(specification_g1)) => 0.5,
    :Grant_White => SemML(obs_g2, RAM(specification_g2)) => 0.6,
)

The new Sem() and loss-term constructors rely less on keyword arguments and more on positional arguments, but some keywords support is present.

• update_observed!() was removed. It was only used by replace_observed(),
  but otherwise in-place model modification with unclear semantics is error-prone.
• replace_observed(sem, data) was simplified by removing support of additional keywords or requirement to pass SEM specification.
  It only creates a copy of the given Sem with the observed data replaced,
  but implied and loss definitions intact.
  Changing observed vars is not supported -- that is something use-case specific
  that user should implement in their code.
• check_single_lossfun() was renamed into check_same_semterm_type() as
  it better describes what it does. If check is successful, it returns the specific
  subtype of SemLoss.
• bootstrap() and se_bootstrap() use bootstrap!(acc::BootstrapAccumulator, ...)
  function to reduce code duplication
• bootstrap() returns BootstrapResult{T} for better type inference
• fit_measures() now also accepts vector of functions, and includes CFI by default (DEFAULT_FIT_MEASURES constant)
• test_fitmeasures() was tweaked to handle more repetitive code: calculating the subset of fit measures, and compairing this subset against lavaan refs, checking for measures that could not be applied to given loss types (SemWLS).

[Maximilian-Stefan-Ernst]

Suggested change

	In this case, [`FiniteDiffWrapper`](@ref) method to generate a wrapper around the specific `SemLoss` term that only uses its objective
	to calculate the gradient using the finite difference approximation.
	In this case, [`FiniteDiffWrapper`](@ref) can be used to generate a wrapper around the specific `SemLoss` term. This wrapper only uses the `LossTerm`s objective, and calculates the gradient using finite difference approximation.

[alyst]

@Maximilian-Stefan-Ernst It might be a nice idea to use copilot for catching typos, incorrect sentences, but also potential bugs.
I cannot select copilot as a reviewer -- I'm not exactly sure why, whether it is the organization/repository-level setting, or it's my status in the repository.
But I'm also fine if SEM.jl is kept AI-free :)

[Maximilian-Stefan-Ernst]

Is there a reason to pass typeof(term1) instead of term1? I personally find the syntax a bit cleaner without the extra typeof call.

[alyst]

This call calculates chi2 for the entire model (which could be multi-group), I wanted to make it clear that no information from the term1 is used.

[Maximilian-Stefan-Ernst]

Maybe we should throw an error for a Sem with no terms?

[alyst]

I think the Sem constructor should throw an exception if there are no SEM terms.
Returning 0 here seems legit if there is no data, but we can also change it into @assert.
Throwing an exception in situations like this will just add a lot of redundant code.

[Maximilian-Stefan-Ernst]

I think before this was handled inside RAMSymbolic - is there a reason to move it here?

[alyst]

My original idea was to move away from passing the kwargs... around.
It causes various issues, adds maintenance overhead, and interferes with the modularity/extensibility goal.
Instead, the constructors should support their specific keywords only.
This PR still retains kwargs... passthrough for compatibility, but moves all "parameter compatibility" logic to the Sem construction.

With the old logic the user only specifies the implied and loss types, and SEM.jl figures how to tweak their parameters to make them compatible.
RAMSymbolic/SemWLS is one such example, where RAMSymbolic decides how to construct itself knowing how it will be used.
In this PR, the implied constructors are agnostic of the loss functions they will be used in.
The loss constructor will throw an exception if the implied object is not compatible.

In principle the logic here could be removed -- the user should be able to easily fix the SEM construction based on the error message thrown by SemWLS.