CLI11 replacing Boost::program_options.

[lsawade]

Description

Boost::PO is very rigids and manual in terms handling CLI options. This replaces Boost::PO with CLI11 to handle the command line options (even Henry has contributed here!).

Issue Number

Towards #1643

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