rework dependencies and document properly

[mzaiss]

OS, pythin version

etc.

pypulseq versions

seq file versions

[J-Endres]

This is written in pyproject.toml but we we should include it in the docs and also in the README.

Furthermore, as many dependencies as possible should be optional.
Research best practice of how to make it optional in python (e.g.: only define function if import succeeds or import in function body)

Current dependencies, defined in pyproject.toml

dependencies = [
    "ismrmrd",            # used only in a helper for exporting
    "matplotlib>=3.5",    # essential - check version
    "pydisseqt>=0.1.13",  # for pypulseq and .dsv import, pretty small dependency
    "pypulseq",           # used in many scripts, but MR-zero should function without it
    "requests>=2.20",     # for downloading BrainWeb data, should be optional
    "scikit-image",       # maybe used for phantom interpolation? Please check!
    "scipy>=1.7",         # or is this for interpolation? One of those is optional!
    "torchkbnufft",       # nufft reco - is this used directly / can this be optional?
    "torch>=1.12",        # essential - check version
    "tqdm",               # progress bars - probably can be removed (or used properly!)
]