Simultaneous NumPy 1.x and 2.x compatibility

TLDR: Simultaneous NumPy 1.x and 2.x compatibility, modernized building/testing/documenting, and the Python ar module now supports keyword arguments.

Python & NumPy Compatibility:

• Dropped Python 2.x, 3.7, and 3.8 support (#45, #48)
• Updated NumPy build process to support both 1.x and 2.x
• Added NumPy 2.x compatibility to Python bindings (#35, #40)
• Added keyword argument support to ar.arsel() (#37)
• Added Python test suite for AR module (#34)

Build System & Versioning:

• Migrated from Travis CI to Circle-CI (#31, #32, #33)
• Added version flag to C++ arsel application (#43)
• Added CXXFLAGS to arsel version output (#47)
• Added __version__ observable to Python ar module (#49)

Documentation:

• Fixed pyproject.toml license field (#42)
• Deployed Doxygen documentation to gh-pages (#50, #52, #53)
• Updated Doxyfile from version 1.8.1 to 1.9.8 (#55)
• Addressed several Doxygen warnings (#59)
• Enabled MathJax rendering with modern CDN (#61)
• Updated links to documentation

Bug Fixes & Code Quality:

• Fixed C++17 deprecation warnings in ar.hpp (#44)
• Fixed AR_POCHHAMMER arity issue in FSIC YuleWalker implementation (#57)