add support for multiple OBSERVATION_IDs

[o-smirnov]

@Ermiasabebe has a KAT-7 MS that contains multiple observations of the same object stuck together. These are differentiated by the OBSERVATION_ID column. It is an easy matter to process them one by one by setting the TDL option ms_sel.ms_taql_str = OBSERVATION_ID==N. However, it would be very nice to add explicit observation ID support to Pyxides.ms and Pyxides.mqt (as is already done for DDID and FIELD).