Provide tool for determine contrast match point

[butlerpd]

since the strength of neutrons is in the contrast matching ability, a simplified tool for calculating that based on several (2 or more — though an "idiot warning" should probably be generated if less than 4?) data sets. There should be the usual PrView? and Invariant option for estimating the background and several options for determining I which should include choosing a Guinier fit to get I0 or calculating an integrated intensity within a Q range (which also covers the case of choosing the lowest q point). The tool should then provide the plot and compute and print the estimated match point.