fix: heatmap plot colorbar and internal refactoring

[thatlittleboy]

Overview

Description of the changes proposed in this pull request:

• Fixed the colorbar aspect ratio in shap.plots.heatmap, see:
  
• Internally, I've switched to an ax-based implementation instead of plt.imshow(), plt.barh() etc.; primarily so we don't have to do pl.gca() all over the place, and also the axes is needed to pass into plt.colorbar() to avoid warnings about

     Unable to determine Axes to steal space for Colorbar. Using gca(), but will raise in the future. Either provide the *cax* argument to use as the Axes for the Colorbar, provide the *ax* argument to steal space from it, or add *mappable* to an Axes.
  

• The keen eye might notice that in the baseline images above, the tick directions have swapped from "in" to "out" -> note that the tick directions being "in" in the original baseline image is just a result of pytest-mpl using the "classic" matplotlib style; anybody with a more recent matplotlib version should already have the tick directions being "out" even prior to this PR. I only made this explicit in-code so that the baseline images generated by pytest-mpl reflects what most users would get by default ("out" tick directions).
• Render the f(x) text in MathTex rather than plain text to match with the force plots.
• Added comments and grouped similar code (e.g. the tick/axes manipulation code) together for better code readability.

Checklist

• Closes #xxxx
• All pre-commit checks pass.
• Unit tests added (if fixing a bug or adding a new feature)
• Added entry to CHANGELOG.md (if changes will affect users)

[codecov]

Codecov Report

Merging #3040 (3745d13) into master (198dcb6) will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master   #3040   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files          90      90           
  Lines       12855   12850    -5     
======================================
+ Misses      12855   12850    -5     

Impacted Files	Coverage Δ
shap/plots/_heatmap.py	0.00% <0.00%> (ø)

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[connortann]

LGTM! There are several notices from Codecov about patches not covered by tests, as the code coverage is not currently being reported correctly. I'll see if I can look in to that tomorrow.

[thatlittleboy]

Failing tests are from pytorch mnist cnn, irrelevant to the changes in this PR. See #2960