Temporarily allow tei:locus without attributes

[wlpotter]

@dlschwartz We recently constrained tei:locus to require an @from or @to attribute, but we are not going to have a chance to fix all of the validation errors prior to the workshop. So we would like to roll back that change, and allow locus to validate even without attributes

We may come back around after the workshop to fix these so they all have attribute values.

[dlschwartz]

I've updated the schema to allow this. Before I close this, however, might we want to keep throwing this error so that the proofreaders can fill in the data? I can easily reestablish this if we want it.