Fixing XPath header behavior

This pull request does the following:

All headers in the resulting index file are now XPaths
--xpaths is now --disambiguate-xpaths. Any unique XPaths will be changed into a shortened version.
Information within the index file is no longer missed!
New function: grab_elements(), for your XPath comparison needs. Returns a tuple of the element names in the order they appear in the XPath.
covert_xpath_to_tag has been removed, since it's functionality is no longer needed.

SETI / rms-pds4indextools