Proposed Changes

Adapted the reading of the higher grids, such that it now uses the various meshreader classes. There is some additional functionality, namely that it is now possible to use rectangular and box grids with the higher order FEM solver, but it is mainly meant to make the reading more consistent with the code infrastructure. For this reason additional base classes have been introduced for the reading of CGNS and SU2ASCII files.

Related Work

This is the first of a couple of PRs to clean up the code for the DG-FEM solver. It does not add any functionality nor it fixes any bugs.

PR Checklist

Put an X by all that apply. You can fill this out after submitting the PR. If you have any questions, don't hesitate to ask! We want to help. These are a guide for you to know what the reviewers will be looking for in your contribution.

• 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.