Add sampling option to heatmap widget.
Merge pull request #142 from astaric/heatmap-on-sample

Add sampling option to heatmap widget.
Added color scale options.
Time-based sampling on heatmap.
Insure at least some coloring for non zero cells.
Merge pull request #144 from astaric/time-based-sampling

Time-based sampling on heatmap.
Use the actual maximum contingency value for scaling factor.

Before the maximum value in the root was used.
Change sampling options.
Undo change to n_bins.
Merge pull request #145 from ales-erjavec/heatmap

Heatmap color scale
Revert to no outlines.