frequency: either floating point GHz or Hz or a string with units if set to zero, then use the mean of all ref_frequencies
spw: integer, modify the REF_FREQUENCY for this spw only
Example:
CASA <27>: au.modifyReferenceFrequency('uid___A002_X93514d_X220.ms.split.phase_inf_reffreq',frequency=102.901512)
Changing from 103.909325 to 102.901512 GHz