Write function to persist pfsArm file

md = arcExp.getMetadata().toDict()
key = "PFSCONFIGID"
if key in md:
pfsConfigId = md[key]
else:
self.log.info('No pfsConfigId is present in postISRCCD file for dataId %s' %
str(dataId.items()))
pfsConfigId = 0x0

pfsConfig = butler.get("pfsConfig", pfsConfigId=pfsConfigId, dateObs=dataId["dateObs"])

pfsArm = spectrumSetToPfsArm(pfsConfig, spectrumSetFromProfile,
dataId["visit"], dataId["spectrograph"], dataId["arm"])
butler.put(PfsArmIO(pfsArm), 'pfsArm', dataId)

should be encapsulated as a function as it is the standard way to persist a pfsArm file.