Fixed test125cells when using the Wendland kernels. Neighbour number tolerance was not large enough.

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

Removed explicit references to p->rho_dh in the test27 and testPair unit tests as these might not exist in all hydro schemes.

Equation of state

This implements @tt's suggestion (#195) to allow for changes in the equation of state of the gas. The transformation between thermodynamic variables are now all located in one single file. Users can supply a different set of transformations if they want to simulate a different kind of gas. The SPH schemes themselves remain unchanged.

This is "simply" a harmless re-factoring of the code gathering common equations into a single file. It passes all the tests but you may want to cross-check things. Thanks!
Branch can be removed.

See merge request !215

Removed the read of the signal velocities in the vectorized DEFAULT_SPH routines as these quantities are not used any more.