Add alternative to rdfs:label for giving python names

[rpgoldman]

I'm not really a fan of us hijacking the rdfs:label property -- which is meant to be a human-readable name -- for the purposes of making a python-name.

Thinking of how we might do this without disturbing existing code, how about adding an annotation property like sbol:pythonIdentifier and having the sbol_factory first look for a value of pythonIdentifier and falling back to rdfs:label if it can't find a pythonIdentifier?

I was just worried that existing tools might want to use rdfs:label for their purposes and wouldn't be expecting it to be used in the way we are using it.

[rpgoldman]

Jake points out, apropos of my backwards compatible suggestion that

We're also at an alpha state without many copies in the wild, and thus don't need to worry too much about backwards compatibility.

So we could actually replace the use of rdfs:label instead of shadowing it. That would be up to you all.