Better function name for the first initialiation of all the parts at the very start of engine_init_particles()

Add chemistry in part

See merge request !486

Add the possibility to read basic chemistry information from the parameter file and propagate this onto the particle at initialisation time. More uniform naming convention for the first initialisation of properties.

Add a function that returns the name of a specific chemistry element. Use that function to print the names in the snapshot header.

Make the enum containing the various chemical species a common feature accross all chemistry models. Fixed typo and documentation.

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

Better way of dealing with the chemistry i/o. The i/o routines directly call the list construction, lowering the burden on the hydro scheme mainteners.

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

This reverts commit d6acb7a1.

Meant to add to another branch...