Improve cvt_archive_heatmap flexibility

[btjanaka]

Description

This PR makes cvt_archive_heatmap more flexible by adding an ec parameter to control the edge color of the polygons.

We also rearrange several existing arguments to more closely mirror grid_archive_heatmap — specifically, we move plot_samples, plot_centroids, and ms to the end of the argument list. This reordering should not break anyone since we require keyword arguments for this method by putting a * in the signature.

Regarding style arguments for samples and centroids: I have decided to leave the sample and centroid style arguments (i.e., the arguments to ax.plot) as is for now, as it seems pretty rare to plot centroids/samples, so the added complexity of making them configurable may not be worth it. We can always add this later if there is a need for it.

This PR also fixes several bugs encountered while making these API changes.

TODO

• Add ec parameter for controlling color of Voronoi cell boundaries
• Test style for Voronoi cells (ec and lw)
• Rearrange parameters to line up more closely with grid_archive_heatmap
• Make background color of empty cells be transparent — this is important when someone wants to save a transparent image — in this case, we don’t want the background to be white
• Throw an error if attempting to pass plot_samples when the archive has no samples
• Fix bug with division by zero error when min_obj and max_obj are identical — since we normalize by max_obj - min_obj , we were getting such errors if min_obj equaled max_obj

Questions

Status

• I have read the guidelines in
  CONTRIBUTING.md
• I have formatted my code using yapf
• I have tested my code by running pytest
• I have linted my code with pylint
• I have added a one-line description of my change to the changelog in
  HISTORY.md
• This PR is ready to go