feat: make matplotlib core dependency and refactor visualization code

[alexander-held]

This turns matplotlib into a core dependency. As a result, contrib.visualize_matplotlib is moved into submodules of visualize instead, with the functionality split up into three new components:

• visualize.plot_model (visualize fit model: data/MC, templates),
• visualize.plot_result (visualize inference results: correlation, pulls, ranking, scan, limit),
• visualize.utils (utilities, currently just figure saving).

The change allows for some simplifications in the visualize API, where the method kwarg is dropped.

Breaking changes:

• method kwarg removed from visualize API
• contrib.matplotlib_visualize no longer exists, functionality is split up into visualize.plot_model, visualize.plot_results and visualize.utils

* matplotlib is now a core dependency
* breaking change: method keyword argument removed from visualize API
* breaking change: contrib.visualize_matplotlib moved to visualize submodules: plot_model, plot_result, utils
* added missing modules to API documentation

[codecov]

Codecov Report

Merging #250 (1cd2433) into master (38047ff) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #250   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        17    +2     
  Lines         1728      1720    -8     
  Branches       269       261    -8     
=========================================
- Hits          1728      1720    -8     

Impacted Files	Coverage Δ
src/cabinetry/visualize/__init__.py	100.00% <100.00%> (ø)
src/cabinetry/visualize/plot_model.py	100.00% <100.00%> (ø)
src/cabinetry/visualize/plot_result.py	100.00% <100.00%> (ø)
src/cabinetry/visualize/utils.py	100.00% <100.00%> (ø)

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