Merge branch 'cleanup_rho_dh' into 'master'

Cleanup rho dh and use of the EoS functions

A few more rather unimportant cosmetic changes:

 - moved rho_dh to density.rho_dh everywhere.
 - added a force.f term to contain the transformed value of rho_dh. 
 - When computing the sound speed, I am now using the functions defined in equation_of_state.h everywhere.

The particles are the same size in memory as before despite the change. I had to update the tolerances for the tests as rho_dh changed meaning. 

See merge request !255