Flavor --> flavour and better naming convention for the functions writing cooling and chemistry info to snapshots.

Grackle chemistry --> GEAR chemistry. Moved the output of subgrid model to a separate part of the HDF5 file.

Collect the radiated energy of all particles when computing statistics and output the result in the log file.

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

Use the cooling time-step condition when computing dt. Better documentation. More uniform interface accross cooling functions.

Added the new cooling parameters to the parameter documentation. Renamed the parameters to differentiate them.