feat!: Implement covariance-based impact calculations

[MoAly98]

This closes #513. Replaces previous PR (#515) due to history mess.

To-do for this:

• save impacts method used in ranking results as metadata
• create alternative-looking plots for method
• CLI support for method specification
• Add pure statistical uncertainty impact in impacts summary

Expected missing coverage in this PR since they are implemented but not exposed in API:

• Impacts per modifier type are not tested
• Impact summaries are not tested

These should be addressed in a follow-up PR where the API gets exposed and tests are added. This can probably be a PR on its own where we return a grouped impact table/plot.

* BREAKING CHANGE: default ranking method switches to covariance-based
* add option in `fit.ranking` to specify which impact-calculation method should be used, supporting NP shifting and covariance-based method. 
* Place holder with no implementation added for computing impacts by global observable (aux data) shifting   
* Modularised the computation of impacts using the different methods
* return impacts summary (total systematic/statistical/normfactors/MC stats impacts) from impact calculations
* All impact calculations return the parameter impacts for each type of modifier, as well as the combined list of impacts for all modifier types. 
* The `RankingResults` object now holds information on what impact method was used
* Different plotting aesthetic is implemented for covariance-based impacts
* Ranking plot files now have the impact method used as a suffix
* Impacts computation method can be specified from CLI
* StatOnly impacts are computed and stored in impacts summary

[codecov]

Codecov Report

Attention: Patch coverage is 98.65772% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.90%. Comparing base (1576a4d) to head (f02df78).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/cabinetry/fit/__init__.py	99.21%	0 Missing and 1 partial ⚠️
src/cabinetry/visualize/plot_result.py	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #519      +/-   ##
===========================================
- Coverage   100.00%   99.90%   -0.10%     
===========================================
  Files           22       22              
  Lines         2096     2208     +112     
  Branches       291      313      +22     
===========================================
+ Hits          2096     2206     +110     
- Partials         0        2       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.