Rich testing

[r-leyshon]

A couple of very minor updates to include requirements.txt and a template .gitignore:

• requirements, to help reproduce the environment used to run PPI. I have not pinned the versions but it may be preferable to do this, particularly for numpy and possibly matplotlib where breaking changes are frequently encountered.
• the .gitignore to help prevent the accidental commit of unintended files like .DS_Store on mac. The template comes from toptal which I frequently use to prepopulate gitignores dependent on the os and software used. For this project I selected mac-os, Windows & python-vanilla.