Fix CMFD to work with scipy 1.13

[paulromano]

Description

Recent PRs are failing on CMFD tests due to the release of scipy 1.13. This PR adds a few fixes to get those tests working again:

• The indptr attribute of sparse data structures in scipy 1.13 is now an array with int64, instead of int32. OpenMC C API functions accept int32s, so the indptr arrays need to be cast first.
• Constructing a CSR matrix before would always result in underlying arrays that were sorted by index within a given row, but with scipy 1.13 that doesn't seem to be the case. There is evidently an implicit assumption in OpenMC that CSR data coming in is sorted, so I had to add explicit calls to sort_indices() to get it working (otherwise it fails on the Gauss-Seidel iteration).

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)

[eepeterson]

Thanks for the quick fix @paulromano