[FIX] colorpalettes: fix BinnedContinuousPalette color assignments

[robertcv]

Issue

There is an inconsistency between decimal_binnings and BinnedContinuousPalette on whether the equal sign is on the upper or lower border of bins.
The result is that we get the wrong colors for data points that lie on the border between bins.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4609 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #4609      +/-   ##
=========================================
- Coverage   83.62%   83.6%   -0.02%     
=========================================
  Files         281     276       -5     
  Lines       56194   55389     -805     
=========================================
- Hits        46991   46309     -682     
+ Misses       9203    9080     -123