Merge branch '2d_hydrodynamics' into 'master'

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