Add PureXML.Validation.Schematron: compile a Schematron schema (matching
schema/pattern/rule/assert/report by local name, so the ISO and legacy
namespaces both work) and validate a document. A rule's context XPath
selects the nodes it fires on; each assert/report test is evaluated relative
to the node via the XPath context-node API. A failed assert is an error, a
matched report a warning; within a pattern a node fires the first matching
rule. Adds XPath.Query.nodes(over:) returning matched tree nodes so one tree
can be queried repeatedly.
Closes#52Closes#53Closes#25
