Feature/pypi workflow

[SorooshMani-NOAA]

Add workflow to automatically release pyschism on PyPI on publish tag.

[SorooshMani-NOAA]

This addresses #23 (comment)

@cuill, in order for this to work, you or another admin developer needs to create a PyPI token (https://pypi.org/help/#apitoken) and add it to the repository secrets (GH documentation) with the name PYPI_TOKEN.

Then every time someone creates a release tag, described here, on the repo with a proper tag (e.g. v0.1.1), the workflow will run and automatically uploads the build to PyPI.

Note that I excluded python 3.6 from the build because it had some issues with netcdf-flattener when I tried to build it.

[SorooshMani-NOAA]

@cuill if everything looks fine with this, when you set-up the PyPI token, I appreciate it if you could also create a release tag (e.g. v0.1.1 or anything you see fit) so that pyschism is published on PyPI. I have pyschism as an upstream dependency in coupledmodeldriver and I need pyschism release to be able to release next version of CoupledModelDriver