Extraneous `us` being added to charset names

[gaurav]

Reported as:

The program expects that imported nexus files will be named .nex. If they are named .nexus, it will add the “us” to the locus name. For example: BEGIN SETS; CHARSET L001us = 1-1487; CHARSET L002us = 1488-2507; CHARSET L003us = 2508-4544;