1554 - Move enumerations to core Part II

[icui]

Description

• Rename namespace specfem::connections to specfem::element_connections
• Rename namespace specfem::interface to specfem::element_coupling
• Move 'specfem::wavefieldandspecfem::displaytospecfem::enums`

Issue Number

Closes #1554

Checklist

Please make sure to check developer documentation on specfem docs.

• I ran the code through pre-commit to check style
• THE DOCUMENTATION BUILDS WITHOUT WARNINGS/ERRORS
• I have added labels to the PR (see right hand side of the PR page)
• My code passes all the integration tests
• I have added sufficient unittests to test my changes
• I have added/updated documentation for the changes I am proposing
• I have updated CMakeLists to ensure my code builds
• My code builds across all platforms

[lsawade]

LGTM