Refactor code base and make an installable package named `optbeam`

[DanielKotik]

Two step process:

1. Refactor the multiple optbeam.py files to a pure Python installable (via pip install -e .) package optbeam:

   optbeam
   ├── 2d/
   ├── 3d/
   └── __init__.py
   

2. Enable package build process for Cython using:

   python setup.py build_ext
   python setup.py install --user

   with

   optbeam
   ├── 2d/
   ├── 3d/
   ├── __init__.pxd
   └── __init__.py
   

   see:

   • https://levelup.gitconnected.com/how-to-deploy-a-cython-package-to-pypi-8217a6581f09
   • https://medium.com/swlh/distributing-python-packages-protected-with-cython-40fc29d84caf

[DanielKotik]

See #12 for the first part.