Coloc 2: Add alternative TwinCuror implementation to simplify plugin
This change was suggested by Johannes Schindelin to make the plugin
more simple. Predicates now are only there for the mask and don't handle
thresholding. The PredicateCursor now only drives one cursor and the
TwinCursor drives two PredicateCursors. Up to now only PearsonsCorrelation,
its tests and AutoRegressionThreshold is using this cursor.