Refactor form_matrix method on depletion chain class

[eepeterson]

Description

This PR breaks up the responsibilities of the form_matrix method into separate methods - one for decay and one for the reaction rates. The decay matrix can be created once and cached on the chain so it doesn't need to be recomputed for every material for every step and/or stage in higher order integration schemes. While this provides no real performance benefit by itself yet, it enables other modifications that do and generally improves the separation of responsibility in the depletion module.

Fixes # (issue)
N/A

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) 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 this improvement @eepeterson!