feat: Add shape plots

[matthewfeickert]

* Add plot.shape_hist API
* Make density a kwarg for plot API
* Add plot.shape_hist to examples notebook

[codecov]

Codecov Report

Merging #19 (6ae3dec) into master (f8b3006) will decrease coverage by 6.84%.
The diff coverage is 3.33%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   35.71%   28.87%   -6.85%     
==========================================
  Files           6        6              
  Lines         112      142      +30     
  Branches       17       27      +10     
==========================================
+ Hits           40       41       +1     
- Misses         72      101      +29     

Flag	Coverage Δ
unittests	28.87% <3.33%> (-6.85%)	⬇️

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Impacted Files	Coverage Δ
src/heputils/plot.py	15.31% <3.33%> (-4.44%)	⬇️

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