validate DOM using "location" subset schema

[telic]

The schema includes a concept of "locations" (kinds of documents) where a subset of the defined tags are allowed, but this is not currently enforced anywhere. DOMValidator should be modified (or a new validator created) to only allow tags within a given location subset. The location to match against could either be encoded into the document itself in some way (eg. as an attribute on the root element?), and/or could be provided as a runtime parameter.