Fixes in MicroXS.from_multigroup_flux

[paulromano]

Description

This PR fixes two issues in MicroXS.from_multigroup_flux. First, as described in #3121, when a MicroXS object is created using this classmethod, it cannot be written to csv, This was found to be due to the fact that it was creating a 2D array with shape (nuclides, reactions) instead of a 3D array with shape (nuclides, reactions, group) as it should have been. The other issue I observed is that this method modifies the value of multigroup_flux that is passed in, which can be undesirable for the user.

One other small fix in here is for the IndependentOperator class, which now accepts a normal list of materials (before you had to pass an instance of openmc.Materials).

Fixes #3121

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[pshriwise]

I think the changes above look good, but would this also do the trick?

Suggested change

	return cls(microxs_arr, nuclides, reactions)
	# ensure microxs_arr has the correct number of dimensions for a one-group cross section
	microxs_arr = np.expand_dims(microxs_arr, -1)
	return cls(microxs_arr, nuclides, reactions)

[paulromano]

That would work if we keep microxs_arr as 2D, but it is a lot less obvious to someone reading the code in my opinion, so I'd prefer the current version (explicit is better than implicit).

[pshriwise]

Thanks @paulromano!

[yardasol]

@paulromano Just doing some poking around on this: is MicroXS.from_multigroup_flux() supposed to return a MicroXS object with one group? This is not stated explicitly in the documentation.