Fix periodic parallel preprocessing

[bmunguia]

Proposed Changes

Fixes #2518.

Modify the logic in CGeometry::PreprocessPeriodicComms() to use unordered sets of point-marker pairs when determining number of points to communicate to each destination rank.

When counting points, the old logic just checked that the previous point was not the same as the current point, and when generating the list of points to be communicated, there was no check in place. In some cases, the old logic would lead to a segfault during preprocessing, such as the old result shown below for the isentropic vortex.

Before

After

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.

[bigfooted]

Hi Brian, can you add this testcase to the regression tests as well?