Allow coloring subsets of cells

[kimauth]

Allows to specify a subset of the cells to color. This can make sense especially with mixed grids.
Also adds the docstrings for coloring to the docs.

[codecov-commenter]

Codecov Report

Merging #402 (a7b40e5) into master (5cbe099) will increase coverage by 0.00%.
The diff coverage is 90.90%.

❗ Current head a7b40e5 differs from pull request most recent head 72a8720. Consider uploading reports for the commit 72a8720 to get more accurate results

@@           Coverage Diff           @@
##           master     #402   +/-   ##
=======================================
  Coverage   91.00%   91.01%           
=======================================
  Files          22       22           
  Lines        2847     2849    +2     
=======================================
+ Hits         2591     2593    +2     
  Misses        256      256           

Impacted Files	Coverage Δ
src/Grid/coloring.jl	91.89% <90.90%> (+0.14%)	⬆️

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