Description

This PR enables using external source rates together with transfer rates that specify a destination_material. Previously, the Integrator raised an error if both were set.
Now the following steps are performed:

1. Augment diagonal matrices first by adding the external source column and auxiliary row of zeros
2. Augment off-diagonal transfer matrices by adding extra row/column of zeros when destination or source material has an external source, matching the dimension of augmented diagonal matrices.
3. Assemble the block matrix and solve
4. Reformat nuclide vectors to match the original size.

The implementation comes from a previous patch of Valerio Mascolino (https://github.com/vmascolino-anl).

Other

Documented missing external source rates in methods/depletion.rst and usersguide/depletion.rst

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)