Determine whether nuclides are fissionable in volume calc mode

[paulromano]

Description

This PR fixes a potential issue in sampling external source sites when OpenMC is loaded in volume calculation mode (which is used by the plotter). When running in volume calculation mode, we only load a select few attributes from nuclide .h5 files. Right now, we don't determine whether a nuclide is fissionable. However, for a proposed improvement to our plotter (see openmc-dev/plotter#131), it will be possible to sampling external source sites, which may require information on whether a material is fissionable if the "fissionable only" constraint is applied to the source. Without that information, a source with the "fissionable only" constraint will always be rejected because it won't think any material is fissionable (because the underlying nuclides aren't fissionable). This branch adds a small block of logic to determine if a nuclide is fissionable when it is loaded in volume calculation mode.

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)

[pshriwise]

Tiny suggestion here and then I think this should be good. Thanks @paulromano!

[pshriwise]

Thanks @paulromano!