Add identifier annotation to character definitions in output NeXML

[hlapp]

The NeXML generated by OntoTrace includes taxon identifier annotations for OTUs using the dwc:taxonID metadata attribute. There should also be an identifier annotation for characters (<char/> elements in the NeXML).

The metadata attribute should have strong enough semantics that a consumer can extract the annotation and assume that the value found is indeed the entity or other identifier for the character. A candidate term proposed by @balhoff is IAO:denotes (IAO:0000219), which is a subproperty of IAO:is about (IAO:0000136) and has the following definition:

denotes is a primitive, instance-level, relation obtaining between an information content entity and some portion of reality. Denotation is what happens when someone creates an information content entity E in order to specifically refer to something. The only relation between E and the thing is that E can be used to 'pick out' the thing. This relation connects those two together. Freedictionary.com sense 3: To signify directly; refer to specifically

Perhaps this is strong enough. Tagging @jar398, @arlin, and @rvosa in case they have a better suggestion.

[hlapp]

This is a prerequisite for phenoscape/rphenoscape#2.

[balhoff]

This is implemented and live now.