Fix MG species restart crash

[aureliegrebe]

Proposed Changes

Currently, the species solver crashes when restarting a multi-grid simulation due to mismatched grid levels in two calls in CSpeciesSolver::LoadRestart. This PR fixes that, and modifies a test to exercise this code path.

It'd be great if someone could also double-check whether this code makes sense. Given this bug, I suspect these lines have never been run since they were committed 5 years ago.

Related Work

I'll send an accompanying PR for the new restart file needed for the test case.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.