[r] add poc matrix projection interface

[immanuelazn]

Add an sklearn-like interface for creating pipelines in terms of fitting, projection, and combining operations on matrices.

Current thoughts:

• I deviated from the design docs a little bit to make the inheritance make more sense. I made a default PipelineBase, with Pipeline inheriting form it. I then made a PipelineStep that inherits from PipelineBase. Estimator and Transformer both inherit from this class.

  • PipelineBase and Pipeline differ because Pipelines should have steps. This isn't true for single PipelineSteps
  • PipelineBase and PipelineStep differ to indicate each step has a step_name associated with it. Also to allow for shared interface for transformers/predictors in how they are printed and how they can be concatenated to create a pipeline

• I had to change transform() to project(), given I found a generic base function with the same name. Additionally, I found predict() in the stats package, and changed it to estimate()

• Tests will be added in another sister PR, as there are no transformers/estimators that are built to test functionality here.

• I'm not sure which methods I should provide detail to, given that we are not sure how much of this we want to expose. I provided them to the generics themselves, to allow for a meta-look on how to use methods in both PipelineSteps and Pipeline. However, it isn't clear to me whether I need to continue providing an extensive docstring for every overriden method in child classes.

• I'm not sure which Classes I should be exposing to the reference either. I found that previous BPCells classes (ie IterableMatrix) aren't heavily described in the reference. I provided some information on Pipeline, Estimator, and Transformer, and exposed them to the reference page. I also tried to provide information on how to create a Transformer, and Estimator yourself on the docstring.

• I don't think I'm completely sold on using the show() method as an analog of the python __repr__() dunder. I think it could be more useful to make it act more similarly to what you used to display IterableMatrix, ie where we still have information on what steps are in a pipeline, but also macro information, like hyper params or details on what the step has fit to. In this case, we would have a __repr__() analog somewhere else.

• Probably redundant to have both project() and estimate(). What do you think for just combining them into one?

[immanuelazn]

Closed as we are changing direction to #167