Add checkHNname command.

This is a crab command that intends to do the same as the "check_HN_Name.py" CRAB2 code (I copied most of the code from there). It checks extraction of user HN name from siteDB. But currently I only implemented the "standalone" check mode, because I am not sure what a "crabCheck" mode should do. Anyway, I think it is a good starting point for issue #4141