Pull request for Issue1045 - Add option to fit and use a single curve to REIXSXESImageInterpolationAB

see #1045
ADDED:
curve2Disabled flag to REIXSXESImageInterpolationAB, and changed
computeCachedValues() to revert to original uninterpolated shifting when
curve2Disabled = true

curve2Diable checkbox to REIXSXESImageInterpolationABEditor and modified
editor to remove second curve related lines from shift preview plot,
displaying the one curve used at all points on the plot.
How big is the shift map again?  What I'm saying is that it might be slow to grab each shift map value individually rather than grabbing a whole chunk and then just accessing the QVector.  But if it seems to work fine then that's good enough.