Allow mixed simulations from mixed scenario types

[jessegrabowski]

Previously, it was only allowed to pass one of final_values, final_value_delta, or final_pct_delta to CGEModel.scenario. One is now allowed to pass a mixture of these arguments.

Also add some simple tests of the scenario function

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📚 Documentation preview 📚: https://cge-modeling--29.org.readthedocs.build/en/29/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.68%. Comparing base (3266395) to head (3272686).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   68.01%   68.68%   +0.66%     
==========================================
  Files          24       24              
  Lines        3155     3171      +16     
==========================================
+ Hits         2146     2178      +32     
+ Misses       1009      993      -16     

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