Added a ReadTheDocs for OSeMOSYS Global

[trevorb1]

Description

• Used PyScaffold to update the directory structure for OSeMOSYS Global. This helped in sorting out what files are needed for read the docs, but also allows OSeMOSYS Global to be packaged for PyPI (Issue Setup osemosys_global as an installable Python package and deploy to PyPI #34)
• Added a readthedocs site (https://osemosys-global-test.readthedocs.io/en/docs/).
  • Included in the docs are installation instructions, working examples, model structure details, and the contributors guide.
  • The API docs of the site has been commented out for the time being since we haven't been consistent with docstrings and what not. So getting that sorted will be a separate task.
• Model functionality has not changed

NOTE
The test site is connected to a fork linked to my GitHub account, which works fine for testing. When I tried to link it directly to the repository on OSeMOSYS' GitHub I was running into weird syncing and permission issues. Maybe @abhishek0208 or @tniet could try hosting the site to see if they get the same issues? Thats why the current URL is called osemosy-global-test.

Issue Ticket Number

Closes #70

Documentation

[tniet]

Looks good to me, @trevorb1! If @abhishek0208 is good with things I can click the merge button and we can see what happens?