Consider splitting pyx/pxd files into individual modules

[jkmacc-LANL]

Developing these two files is getting hairy, since they're getting rather long. It'd be easier to work on them as individual files, though it'd be harder to see how I solved coding problems in other classes when developing. cimport statements would have to be updated, as well.

https://cython.readthedocs.io/en/latest/src/userguide/source_files_and_compilation.html#multiple-cython-files-in-a-package

[jkmacc-LANL]

@rad-lanl I think it might be time for this, as our .pyx and .pxd are getting a little unwieldy. This should only effect development, as everything in Python-land would still be imported from .libgeotess. Would this cause you any issues? I'd do it later, after trying to bring in Rob's work...

[rad-lanl]

Are you thinking individual pxd/pyx per class instance? that might cause some difficulty because of inheritance between classes, but if you can get it to work easily I don't foresee any issues on my end