parser: add S_P_{FLOAT,DOUBLE,LONG_DOUBLE} and related s_p_get_* functions
layouts: add S_P_{FLOAT,DOUBLE,LONG_DOUBLE}/L_T_{FLOAT,DOUBLE,LONG_DOUBLE}

leverage the addition of S_P_{FLOAT,DOUBLE,LONG_DOUBLE} in the SLURM parser
in order to provide float, double and long double support in the layouts
framework using the L_T_{FLOAT,DOUBLE,LONG_DOUBLE} types.
layouts/unit: add float, double and long double entity keys

Map the recently introduced L_T_{FLOAT,DOUBLE,LONG_DOUBLE} to the set of entity
keys in order to ease the unit testing of the layouts framework internals related
to these types.
layouts: remove the slurm_ prefix from layouts API functions

it does not provide any information and make things longer than necessary.
layouts: correct a bug in layout packing logic