Broke one of my own rules by doing too much in a single commit: implemented runner_extra_ghost, moved gradient calculations for both gradient types to new gradient loop, fixed bug in engine.c involving the number of links (which is larger with an extra loop), streamlined gradient methods, documented most GIZMO specific files. Second loop works.

Wrote dimension dependent matrix inversion method and corresponding unit test. Started making Gizmo dimension independent.

Hydrodynamics in 2D + New test cases

This adds the ability to run SWIFT in 1D and 2D. There is a large number of hydrodynamics tests that only exist in 2D so we will now be able to run them. This implements #145.

I have also renamed a few of the test-cases to specify the dimensionality of the problem. 

The Gresho-Chan vortex test-case can now be run and give the expected answer (provided the code is compiled with 2d hydro in const.h). The run.sh script runs the code, plots the solution and makes a movie out of everything.

See merge request !216

First seemingly working version of Gizmo hydro. Removed superfluous messages from vacuum Riemann solver.

Added volume to Gizmo/hydro_io. Added volume unit conversion. Isolated gradients calculations in separate files.