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.

First attempt at getting testSymmetry to work. Made sure test prints out useful information if things go wrong. Implemented Gizmo/hydro_debug.

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

Replaced all gamma constants and power of gamma functions by calls to the respective adiabatic index constant/function in exact and TRRS Riemann solver

Added extra adiabatic index definitions to be used in Riemann solvers. Added unit test to check values.

Added GIZMO_SPH option in const.h. Made sure the resulting code compiles. It probably does not work correctly though.

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.