Populate .gitignore from Python template

[sadielbartholomew]

As alluded to in #37, the .gitignore is presently very bare and many files need to be manually ignored when staging for a commit. The simplest way to get a more useful .gitignore without having to think much about what needs to be added, is to copy one from a pre-populated Python template, for example as provided by GitHub itself (now an option when you create a repo to add it, but it wasn't back in the day...) i.e. the file here.

Notably though, from looking at this template I would also like to customise it a little, e.g. to:

• add Sphinx artefacts to be ignored, such as the .rst.txt under /build/_sources/*,
• add data files (mostly netCDF), which we shouldn't generally touch, but might want to play around with for testing and end up writing over with edits, therefore would be good to include to ignore.