Gravity infrastructure

Sorry, this is a bulky one. Not all of the gravity is here but most of the infrastructure is in place. It's getting tricky to maintain this separate so I'd like to merge all the innocuous changes to the master branch.
All of the gravity tasks and the logic is in place. Only the content of some of them needs to be more thoroughly checked for accuracy and I need to merge the FFT task once I am happy with it. 

With this in it should be much easier for me and Bert to re-import gizmo into the master.

Could you stress test this and verify that your usual hydro test-cases still run ? I'll bring gravity test cases in at a later point.

See merge request !212