Allow transfer rates with nuclides.

[yardasol]

Description

This PR adds capabilities to enable TransferRates to work with specific nuclides. While transferring specific nuclides would most likely not occur in a real fuel processing system, it is a useful feature to have for comparison with other codes (i.e. Serpent2's repr card).

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)

[paulromano]

Thanks for improving this capability @yardasol!

[paulromano]

Looks good to me now! @church89 do you want to take a look at this before we merge?

[church89]

Hey @yardasol, looks good thanks. We discussed it in person and I have nothing to add beside this small docstring clarifications.