Support 1D cvt_archive_heatmap

[btjanaka]

Description

This PR adds support for plotting heatmaps of 1D CVTArchives. grid_archive_heatmap was also slightly modified so that yticks are no longer shown by default (since this is a 1D plot, only the xticks matter).

TODO

• Refactor 1D heatmaps into a util -> archive_heatmap_1d
• Call archive_heatmap_1d in CVTArchive
• Test

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