RF: Consistently use matplotlib.colormaps in mpl namespace

[effigies]

A cm.get_cmap() snuck in since #98. To make clear that colormaps is not a local dictionary, I'm switching to using it from the mpl namespace.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 58.30%. Comparing base (977f26a) to head (cc92be4).
Report is 88 commits behind head on main.

Files with missing lines	Patch %	Lines
nireports/reportlets/mosaic.py	50.00%	2 Missing ⚠️
nireports/reportlets/nuisance.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   58.43%   58.30%   -0.14%     
==========================================
  Files          22       22              
  Lines        2086     2084       -2     
  Branches      364      321      -43     
==========================================
- Hits         1219     1215       -4     
- Misses        783      786       +3     
+ Partials       84       83       -1     

Flag	Coverage Δ
unittests	58.30% <70.00%> (+0.02%)	⬆️

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[jhlegarreta]

Codecov failure is unrelated. Merging.