Accept models with units with same names as other elements

[luciansmith]

Because SBML has a different namespace for units, it is possible to get a model with a unit definition that's the same name as (say) a parameter. The same routine that cleans up other names (i.e. 'compartment' -> 'compartment_') needs to also transform unit definition names to something like 'oldname_unit'.

Reported by: luciansmith