Can `Location` be extended?

[tcmitchell]

See SynBioDex/SBOL-utilities#200

It seems like this is a good candidate for an extension. But @jakebeal says:

Because Location is an abstract class, it's not clear whether any library that isn't using the extension can meaningfully instantiate a generic Location. In pySBOL3, for example, there is no builder registered for Location, so extending Location results in errors when working with a document with pySBOL3 (i.e., without the extension loaded).

That's unfortunate. Is this the way it should work? Can anything reasonable be done about this?

[jakebeal]

In fact, I think this may be an issue to raise at the level of SBOL-specification and at HARMONY