Add predefined diverging colormaps

[kimikage]

This adds 5 diverging colormaps ("BrBG", "PiYG","PRGn", "RdYlGn", "Spectral" colormaps.

Since these are not identical to ColorBrewer definitions, you should use ColorSchemes.jl or similar in your final publication.
In other words, these are more like garnishes than practical items.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.83%. Comparing base (5a282ed) to head (bdddbaa).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
+ Coverage   98.76%   98.83%   +0.07%     
==========================================
  Files          10       10              
  Lines        1293     1293              
==========================================
+ Hits         1277     1278       +1     
+ Misses         16       15       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[kimikage]

Note that "PuOr", "RdGy", and "RdYlBu" could not be reproduced due to asymmetry or discontinuities.

The "PuOr" case is somewhat similar, but it doesn't capture the contrast between the original light orange and dark purple.