Convert Spheral PYB11Generator bindings to produce a single Python module with each package as a submodule

[jmikeowen]

Summary

• This PR is a refactoring/feature
• It does the following:
  • Uses the new submodule feature of PYB11Generator to bind all of Spheral's packages as submodules of a single Python module (SpheralCompiledModues)
  • This allows us to have a single shared library for the final module, with all submodules compiled as static libraries.

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ToDo :

• Annotate RELEASE_NOTES.md with notable changes.
• Create LLNLSpheral PR pointing at this branch. (PR#)
• LLNLSpheral PR has passed all tests.