Add scm runtime versioning and change to a runtime dependency on GMTK

[EricR86]

This pull request addresses some issues with current Segway distributions and adds some features.

• Addition of setuptools_scm to enable runtime versioning (no hardcoded values) based on last Git tag and current commit.

  $ segway --version                                                                                                                                                                                        segway 3.0.2.dev0+g41b154e.d20200730 

• Removes GMTK as an installation-required dependency, and instead does the check after option parsing.
  • Allows for segway --version and --help to run without a dependency on GMTK
  • Allows for installation without GMTK required
  • Allows for acquiring the current Segway version without GMTK being installed
  • Allows to be built on readthedocs without GMTK or a hardcoded version
  • Allows for distribution builds without GMTK
• Fixes missing non-python files both through MANIFEST.in on a best-effort basis (difficult to maintain longterm) and on setuptools_scm when possible through files tracked on Git (which is automatically updated).
  • setuptools_scm successfully adds all testcases and files into the source distribution
  • This fixes Bioconda issues where the LICENSE was erroneously omitted amongst some other non-python/data files.

[EricR86]

@michaelmhoffman this is ready for your review