Isothermal potential

added some more external potentials to gravity, plus a test for energy and angular momentum conservation in the IsothermalPotential/GravityOnly example.

See merge request !205

Moved calculations for the Balsara switch and P_over_rho from hydro_iact_force and hydro_iact_nonsym_force into hydro_prepare_force and hydro_predict_extra.

Rearranged and replaced: div_v,curl_v->balsara,entropy_dt,pressure->P_over_rho variables of the part struct.

i/o conversion

Would that work ?

See merge request !201

Bug fixes



See merge request !202

Corrected divergence update with minus sign for vectorised versions of the symmetrical and non-symmetrical density functions.