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.
It performs well...  and since the user can drag a slider to change displayXOffset_, it changes and needs to be re-computed continuously even with only one shift curve....  If I'm understanding you.