1554 - Move medium to core Part II: refactor medium_container

[icui]

Description

Inheritance structure:

For point (mostly same as current):

• medium_container::point_properties defines medium-specfic fields like kappa, rho etc. (previously point::impl::properties)
• point::properties inherits medium_container::point_properties with added tags

For domain (refactored to be consistent with point)

• medium_container::domain_properties defines medium-specfic fields like kappa, rho etc. (previously medium::properites::data_container)
• medium::impl::accessor is moved to assembly::impl::domain_accessor
• assembly::impl::domain_properties inherit assembly::impl::domain_accessor and medium_container::domain_properties (previously medium::properties_container)

Issue Number

Adds to #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]

should these function now be in specfem::medium_physics::impl so that
specfem::medium_physics::impl::mass_matrix_component?

Not that it matters, but I feel like if we have impl namespaces we truly would havea a natural flow (thinking also about your assembly::impl changes)