WIP: Fix/nek mesh write default expansions
- Added useDefExpansions flag to xml output.
- useDefExpansions then chooses version of WriteGeometry - if not set then Expansions flag set to false.
- Added documentation on useDefExpansions in UserGuide.
- Added check in MergeDoc in Session reader that looks through all xml (or xml.gz) files for duplicate tags. If 2 found then shows error.
Edited by Ankang Gao