add option to memoize disc_zone_n

configure

src/binary_c_structures.h

src/common_envelope/common_envelope_evolution_BSE.c

src/disc/disc_evaporate_cbdiscs.c

src/disc/disc_failure_mode.c

src/disc/disc_hottest_zone_at_radius.c

src/disc/disc_logging.c

src/disc/disc_macros.h

src/disc/disc_nearest_zone_n.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_structure_functions.c

src/disc/disc_thermal_zones.h

src/disc/disc_zone_n.c

src/perl/scripts2/cbdiscs.pl

src/stellar_structure/stellar_structure_HeStar.c

tbse