replace c->stars.do_stars_sub_sort, do_drift and do_sub_drift with cell_flag_do_stars_sub_sort, do_grav_drift and do_grav_sub_drift, respectively.

replace c->hydro.do_sub_limiter with cell_flag_do_hydro_sub_limiter. also use uint16_t for the sort masks.

add an enum with the different cell flags (basically whatever was a char type and looked like a binary flag). will start replacing the cell values in the following commits.

Basic infrastructure for black holes

See merge request !784

Do not reallocate the full sort arrays every time we sort.

Closes #570

See merge request !786

Crash gracefully on start-up if the user compiled with cooling but forgot the runtime flag.

Closes #571

See merge request !785

Added basic support for black hole particles. Particles can be read in, attached to gpart, integrated forward in time, drifted and written to snapshots.

Update h_max

See merge request !768

Added LGPLv3 to top-level of repo.

See merge request !730

Variable AV fix | Cosmology

See merge request !765