Closes #193

[ludivintchokote]

• Added help-implemented.rst
• Updated pytentiostat.py to return help message

[sbillinge]

@ludivintchokote thanks so much for your PRs we are really happy and grateful. For the --version and --help features, I posted a quick PR (#215) which is what I had in mind. Please could you review it for me, and then I can merge it and I will close these two. I didn't write any tests because we don't normally test the CLI and the parser, though I guess we could.