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Between binarywave and record.variables, there was a good deal of
parsing that was conditional on previously parsed data.  Rather than
continue writing spaghetti code to handle each specific case, I've
taken a stab at a general framework for updating structures during
parsing (DynamicStructure and DynamicField).  The docstrings should
explain how they work.  The implementation still has a few holes, but
it works on each of the files in my test suite.

This makes more sense than storing them as a tuple.

Also, use wave_record.wave_info['bname'] as the name for storing the
wave in the filesystem view returned by packed.load.