Have means to indicate that locus has been scanned but no allele identified

[kjolley]

An allele designation of '0' can be used to indicate that a locus is missing from the strain, but we cannot differentiate whether we expect it to be missing, or if it is just missing from an assembly for technical reasons. It would be useful to be able to show separately that a locus had been scanned by a curator but no designation could be made.