initial colorbar functionality

[yvesmauron]

Implemented:

• [x] Possibility to set columns for fields in function call, ordered by x and y.
• [x] Possibility to set categorical style (background color) with color palette
• [ ] Possibility to set annotation (postponed until later, see issue)
• [x] Empty fields need to be shown and should be white.
• [x] Colorbar with custom range per category (instead of annotation)

[codecov-commenter]

Codecov Report

Merging #11 into master will decrease coverage by 0.24%. The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
- Coverage   95.88%   95.63%   -0.25%     
==========================================
  Files           3        3              
  Lines         267      275       +8     
==========================================
+ Hits          256      263       +7     
- Misses         11       12       +1     

Impacted Files	Coverage Δ
R/catmaply.R	95.78% <91.30%> (-0.77%)	:arrow_down:

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