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.

Fixed a horrible error in the vacuum generation condition for all Riemann solvers. Implemented round off tolerant symmetry tests. testSymmetry now runs for GIZMO_SPH.

Increased the number of symmetry tests per solver to cover more cases. Isolated the vacuum Riemann solver into a separate file.

Removed rho_dh from Gizmo/hydro_part. Commented out USE_GRADIENTS and PER_FACE_LIMITER in Gizmo/hydro_iact.