Relax the VOTable requirements when parsing units

When reading VOTables, at least in the 'pedantic=False' mode, it would be useful to remove the 'cds' argument when unit strings are parsed using the Units framework, so that unit strings not compliant with the cds 'dialect', but still parseable by the framework, would not result in an UnidentifiedUnit instance.

I modified the astropy/io/votable/tree.py code in my working copy to do so, and it required only very few trivial tweaks.