Include which arms should be used in the pfsDesign/pfsConfig file

I don't think that we have a way to specify and track which arms are to be/were used to expose a given set of fibres. If I'm right, we should add this information to the pfsDesign (and thus pfsConfig) file. The easiest way would be to add it to the PDU along with the RA and DEC (and ROTANG DAMD-100) as e.g. ARMS = 'brn'
This is sufficient for downstream users to figure out what we did, but as a courtesy we should add this information to the metadata for pfsMerged and pfsObject files too.