Deprecate CUSP scheme and make it an error to use MUSCL_FLOW=YES with centered schemes

[pcarruscag]

Proposed Changes

I never found this scheme to be useful (not an improvement in either accuracy or robustness) and it is not used in any tests.

MUSCL_FLOW= YES is currently ignored for centered schemes, if we ever want to change that it would lead to backward compatibility issues, since we now have a chance to break it, I'm disallowing setting MUSCL_FLOW= YES with centered schemes.

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 have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.

[WallyMaier]

LGTM. I have had terrible success with CUSP as well.