Implement validation for BIDS HED schema version specifications

[happy5214]

Currently, there is a stub named validateSchemaSpec that was intended to validate schema specifications (i.e. nickname/library/version specs), probably in the already parsed form of SchemaSpec objects, but nothing calls it and it currently only does an object type check. What would need to be done to implement this?

[VisLab]

I think this might be meant for the implementation of lazy schema loading --- where users can specify an arbitrary number of partnered schemas without prefixes and they are loaded one at a time if no conflicts. The loader must raise an exception if a conflict occurs and the validation should not proceed.

[happy5214]

This stub pre-dates partnered schemas by several months, but it could be repurposed for that. We'll discuss in our meeting.