Merged requested to merge cjane/nektar:ticket/181-error-checking-in-expansions into master
DefaultVar now is generated only when there is no FIELDS attribute. When DefaultVar exists, it is used as the default for any variables not explicitly listed in FIELDS. The end result is that m_expansionMapShPtrMap has an entry for all variables listed in the VARIABLES section. The necessary warnings and error messages have been added.
The documentation of the user guide includes this new logic in the Expansions section.
Fixes #181 (closed) and #271 (closed)