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.

Updated 'make check' scripts with newer tolerances for the Intel compiler and made the scripts choke if files are missing

First initalisation of particles now happens in space_init() and not in engine_init_particles() any more.