add github actions for releases on push to master

[bch0w]

One of the slow points in releasing packages under adjTomo is that there are multiple tasks that still have to performed manually after an official version push to master, including testing code, releasing to PyPi, pushing and tagging a a release on GitHub.

It would be useful to institute GitHub actions that would take care of these tasks automatically upon push to master. PySEP can be used as the testbed and when there is a system that works I will roll out the functionality to other adjTomo packages.

Useful reference: https://github.com/marketplace/actions/tag-release-on-push-action