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.
Each call is grabbing 64 values (one for each row), so not very big.  Since each value needs an operation performed on it (+ displayXOffset_), I intuitively feel that grabbing the whole vector at once and then adding displayXOffset_ to each value in the vector would not be much faster...  But I also know that optimizing code is often far from intuitive...