Migrate to SciPy sparse arrays

[paulromano]

Description

In recent versions of scipy, they have introduced sparse arrays (e.g., csc_array) that generalize the existing sparse matrix classes to N dimensions. The scipy documentation recommends migrating to using sparse arrays rather than sparse matrices. This PR completes that migration for OpenMC, preferentially using sparse arrays for scipy 1.15+ (when they became fully supported) and falling back to sparse matrices for earlier versions.

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)

[shimwell]

I guess another option is to change the pyproject.toml to ensure scipy 1.115.0 or newer is installed.

[paulromano]

@shimwell We follow SPEC 0 with respect to minimum versions required, which currently says we should support scipy 1.12+