Add XTable.drop_metadata

[jeromekelleher]

It's useful to drop metadata on tables. The only question really is how to deal with schemas, how about:

def drop_metadata(self, keep_schema=False):
     """
     Drops all metadata in this table. By default, the schema is also cleared, except if ``keep_schema`` is True.
     """

[benjeffery]

Makes sense to me - more likely to be issues of confusion if the default is to keep the schema.