Take sas-id from filename instead of ms metadata
The filenames for the plots contain the sasid. This sas-id was taken from the LOFAR_OBSERVATION_ID
attribute in the measurement set, but it turns out this points to the predecessor if there is one.
Instead we should put the sas-id of the current data in the filename. I was unable to find anything like this in the measurement set, so instead I decided to parse the filename to get the current sas-id.