add function to determine the hottest zone at a given radius

src/disc/disc_bisection_rooter.c

src/disc/disc_extend_powerlaw_from_reference_point.c

src/disc/disc_function_macros.h

src/disc/disc_hottest_zone_at_radius.c

src/disc/disc_logging.c

src/disc/disc_new_zone_list.c

src/disc/disc_new_zone_radii.c

src/disc/disc_parameters.h

src/disc/disc_prototypes.h

src/disc/disc_rezone.c

src/disc/disc_show_thermal_zones.c

src/disc/disc_structure_functions.c

src/disc/disc_thermal_zones.h

src/disc/disc_trisector.c

src/disc/disc_zone_crossing_radii.c

src/disc/disc_zone_n.c

src/nucsyn/nucsyn_parameters.h

tbse