Add pfsDesign "variant" field, to be added to the SHA.

We sometimes want to distinguish between pfsDesigns which otherwise might have the same SHA. Two cases are when testing/changing the fibertypes for a given static design, and for distinguishing between designs with sub-arcsec offsets between them.

The proposal is to add a optional VARIANT field to the design which would be incorporated to the SHA. If the field does not exist (as for all existing pfsDesigns), the SHA would be unchanged.