invariant - MVP

want to bring invariant up to the level of corfunc

slider from corfunc

• use the guinier/porod slider
• currently, number of points used - discrete instead of continuous (?)
• currently, invariant uses the last data point as the porod end
  • can add a new porod end point, by default the last data point

add calculation of SLD contrast from volume fraction

• need to change the widgets