Reimplement kernels

[Rohit-Kakodkar]

Description

Please describe the changes/features in this pull request.

This PR reimplements element, sources and receiver kernels for the newer assembly formulation

• Major changes
  • element, source, receiver classes are now exclusive method classes. This is a pre-cursor to replacing element, source and receiver classes to templated functions. This will simplify the interface.
  • Coupled interfaces are templated on medium types and not on domains. Simplifies the interface.

Issue Number

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Checklist

Please make sure to check developer documentation on specfem docs.

[x] I ran the code through pre-commit to check style
[] My code passes all the integration tests
[x] I have added sufficient unittests to test my changes
[x] I have added/updated documentation for the changes I am proposing
[x] I have updated CMakeLists to ensure my code builds
[x] My code builds across all platforms

[Rohit-Kakodkar]

Does not compile on GPUs.