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.

Gravity infrastructure

Sorry, this is a bulky one. Not all of the gravity is here but most of the infrastructure is in place. It's getting tricky to maintain this separate so I'd like to merge all the innocuous changes to the master branch.
All of the gravity tasks and the logic is in place. Only the content of some of them needs to be more thoroughly checked for accuracy and I need to merge the FFT task once I am happy with it. 

With this in it should be much easier for me and Bert to re-import gizmo into the master.

Could you stress test this and verify that your usual hydro test-cases still run ? I'll bring gravity test cases in at a later point.

See merge request !212