Equation of state

This implements @tt's suggestion (#195 (closed)) 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.