sbol.xsd needs a custom xsd:string restriction for the DNA sequence

[drdozer]

The datatype for the DNA sequence string needs to be a custom restriction on xsd:string that restricts characters to those that are legal for IUPAC.

[TrevorFSmith]

I used that xsd to create the original Java classes and it was out of sync with the classes so I removed it from the tree. Once things settle a bit we'll look into writing a correct xsd.

That said, you're right that the classes should restrict the sequence string so I'll leave this open.