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.