Check whether particles or cells are active via an inlined function rather than via a direct read of engine->ti_current

Safer versions of the error() and message() macros that can be used in if/else statements without braces.

Move rho_dh into the density sub-structure. Update the pressure and sound-speed after kick and drift

Moved the structure related to cooling into a separate file. Added a structure to the xpart to contain the accumulated radiative information.

Replaced all calls to fminf and fmaxf by the macros that was added for the integer min/max functions.