Namespaces must not end with a slash

[jakebeal]

It turns out that Java doesn't believe the same things about URI equality that Python does, so SynBioDex/SBOL-specification#476 is going to be resolved differently, saying that string equality is the way that we need to compare URIs.

The key consequence for this is that namespaces will need to be normalized to have no slash at the end, and this will need to be added to the validation rules.