Allow scaled dimensionless units for CDS format

Following an e-mail request for clarification to François Ochsenbein of CDS, Strasbourg, it became clear that the CDS format does allow for scaled dimensionless units, unlike what was assumed so far, with those either having no unit (just a factor) or a simple '-' at the end. This PR implements that (and adds test cases).

(@mdboom: this adds '*' -- standard ascii times sign -- instead of unicode as an alternative for 'x' in the number in front; I'll verify with François that that is what he meant.)