Initial validate.py script.
Make validate.py more amenable to on-line validation.
Validator works pretty well. Now to add tests.

Also change xigt.core to throw a warning instead of an exception for a duplicated ID (only the first will be indexed).

And fix a bug in examples/odin/kor.xml found by the validator (woo!)
More checks for validation.
Make tests more functional (fewer side-effects).
Add more alignment expression validators.

Also add the get_alignment_expression_spans() function, and make get_content() more robust to missing targets.
Add validation check for unspecified refattrs.