feat: yield tables

[alexander-held]

see #158

This adds yield tables. The tables can be visualized with the tabulate package, which is now a core dependency.

The yield tables are currently printed when creating pre- and post-fit data/MC visualizations. Their creation requires the same calculation (in their current format), so it makes sense to include them there. This might be refactored in the future, so for now everything in the new tabulate module is treated as non-public API.

Other considerations:

• could also do a table with yields per channel instead of per bin, this requires an additional calculation where all histograms in the model are concatenated (Summing bins per channel pyhf#1198) -> future PR
• could also add per-sample uncertainties, which also requires an additional calculation (maybe could be integrated into model_utils.calculate_stdev without significant overhead) -> future PR

Small changes:

• model_utils.calculate_stdev returns a list instead of an awkward array

[codecov]

Codecov Report

Merging #159 (b723510) into master (fc984e4) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage   97.96%   98.00%   +0.04%     
==========================================
  Files          13       14       +1     
  Lines        1471     1507      +36     
  Branches      211      219       +8     
==========================================
+ Hits         1441     1477      +36     
  Misses         28       28              
  Partials        2        2              

Impacted Files	Coverage Δ
src/cabinetry/__init__.py	100.00% <100.00%> (ø)
src/cabinetry/model_utils.py	100.00% <100.00%> (ø)
src/cabinetry/tabulate.py	100.00% <100.00%> (ø)
src/cabinetry/visualize.py	100.00% <100.00%> (ø)

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