switch to using a 32-bit flag field and atomics, replace '|=' with '= ( ... || ... )', which should do short-circuit evaluation, and is easier to read.

Don't pass enums in the `cell_*_flag` functions, but rather use the type of the `flags` value itself. Using the enum causes problems when passing the union of more than one flag.

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.

Eagle stellar evolution matthieu

See merge request !787

Correct the external potential help string to not include the now-defunct softened isothermal potential.

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