Fixed data seperation bug.

A PLOT_GROUP column with only NA values was interpreted as group values,
but without proper data handling downstream. This is now handled
correctly.
ScatterPlot had hardcoded restriction for plotting

The plot had hardcoded restriction of only 20 groups. Recoded handling
of point-shapes in plot to dynamic range.
Added data requirements check before Fisher test

Before, a single group without proper amount of levels would cause
exception for entire analysis. This is now caught.