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.