#15 Validate VOEvent 2.0 Tables

[Bob Denny]

The VOEvent 2.0 schema does not permit schema-based validation of What/Table constructions. Specifically, per the specification there must be a Field element which corresponds to each <TD> element within the TR row(s). Add logic to parse and chek each What/Table tree, validating all of the things that the Field elements specify for each of the corresponding TD elements in each row. This could get complicated!