Sets up osemosys_global as a Python package

[willu47]

This establishes the basic framework for osemosys_global as a Python package and for publishing documentation to read-the-docs (closing #34).

• Note that installing doesn't yet do anything, as the scripts inside src are not callable (see Plexos function #37) for a start on this.
• The contained data won't be installed with the Python package, which means that either relative paths to data in the code will need to be changed to fetch data from an online resource; or we package smaller data with the code (to be installed in the Python path)