cluster outlining could be faster

[Thomas Gilgenast]

we recently realized that we could use plt.contour() to draw cluster outlines

this is way faster than the current cluster outliner

this is the approach currently used in e.g. https://bitbucket.org/creminslab/fast3defdr/src/master/fast3defdr/plotting/grid.py

it should be easy to drop into ClusterExtendableHeatmap