Implement filter for cosine of angle of surface crossing

[zoeprieto]

Description

This PL addresses and resolves issue #2640 in the OpenMC repository, which pertains to the possibility of calculating the mu angular distribution of any surface current.
I have added a MuSurface filter to be able to calculate such a distribution when used with a SurfaceFilter.
Currently, the MuFilter only calculates the mu distribution for scattering angles and cannot be combined with a surface filter. On the other hand, the PolarFilter only calculates the polar angle distribution for a plane with the z-normal.
Therefore, this allows us to calculate the mu distribution for a current on any of the OpenMC surfaces.

Fixes #2640

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)

[paulromano]

Thanks for the PR @zoeprieto! Quick design question and then once we resolve that, I can review in full.

[zoeprieto]

@paulromano sorry for the delay, I have already resolved the conflicts and pushing this PR again!

[paulromano]

@zoeprieto Thanks for the updates here! I simplified the implementation considerably by simply inheriting from MuFilter (on both the C++ and Python sides). As you can see, this reduces the implementation in filter_musurface.cpp to <40 lines. I also added a simple regression test to augment what you had already added for a unit test.