Update heatmapping normalizer

[adrhill]

Closes #56.

Breaking change as it renames heatmap keyword argument normalize to rangescale.

[codecov-commenter]

Codecov Report

Merging #57 (ffefe81) into master (8641dfb) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   96.51%   96.42%   -0.09%     
==========================================
  Files          11       11              
  Lines         287      280       -7     
==========================================
- Hits          277      270       -7     
  Misses         10       10              

Impacted Files	Coverage Δ
src/heatmap.jl	100.00% <100.00%> (ø)

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