Optional dependencies

Bug description / feature request:

As described e.g. in https://setuptools.pypa.io/en/latest/userguide/dependency_management.html#optional-dependencies, use

[project.optional-dependencies]
dev = ["pytest", ...]
doc = ["sphinx", ...]

to install the full struphy via pip install struphy[dev, doc]; the command pip install struphy will just give you a minimal version for running code.

Expected behavior:

See above.

Proposed solution:

See above. One thing to test is how an install from source will work in this case, e.g. if pip install .[dev, doc] is necessary/possible.

We can use the branch https://gitlab.mpcdf.mpg.de/struphy/struphy/-/tree/remove-support-for-numpy2?ref_type=heads for this, right @maxlin? I will start editing this branch if ok.