Obtains the catalogue magnitude of a LCOGT standard _LCOGT_STANDARD_MAGNITUDE_
This primitive is suitable for LCOGT optical imagers.
Processing only occurs when it is time to perform photometry, i.e. when the steering header DO_APHOT is true.
An error occurs when the filter is not one of U, B, V, R, or I.
The standard-star catalogue used is $ORAC_DATA_CAL/landolt_ubvri.dat. An error results when the catalogue cannot be opened.