Issue 278 - Refactor of domain folder.

[Rohit-Kakodkar]

Description

Sorry for the big PR 😣

This PR refactors the domain folder.

• Promotes element_types from impl namespace to specfem::compute namespace
  • get_element_on_device functions are now part of element_types
• Refactor kokkos_kernels so that the functions are using the macros
• promote boundary conditions from specfem::domain::impl namespace to specfem::boundary_conditions
• Delete domain folder and its namespace

Issue Number

closes #278

Checklist

Please make sure to check developer documentation on specfem docs.

• I ran the code through pre-commit to check style
• 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]

Sorry for the big PR 😣

LOL this a big boi