File to allow renaming of headers

[rfrenchseti]

In pds4_create_xml_index, right now a header can be one of two strings:

• The full XPath
• The simplified XPath, assuming it is unambiguous

There are use cases where neither of these is desirable, such when XPaths are ambiguous but you don't want to include the full, long XPaths.

One solution is to allow an optional file for the renaming of XPaths. Its format would be something like:

<XPath>,<new name>