Remove initial dilute nuclides in MicroXS

[paulromano]

Description

This PR removes the dilute_initial option in the MicroXS.from_model classmethod (#2568 already removed dilute_initial elsewhere in the depletion solver). Additionally, I've added nuclides and reactions arguments to MicroXS.from_model that make it easier to customize what nuclides/reactions you want to get microscopic cross sections for. When they are omitted, the behavior defaults to the current behavior where the nuclides/reactions are determined from the depletion chain.

Checklist

• I have performed a self-review of my own code
• [ ] I have run clang-format 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)

[yardasol]

Hi @paulromano,
Good work on this nice short change. I just have one question w.r.t. the tests.

[yardasol]

Good work @paulromano!