Prepare dcegm for multiple deterministic continuous states

[amageh]

• Minor (unrelated) edits to documentation.
• Adjust interface to handle multiple upper envelope methods.
• Add assets_begin_of_period for new method
• Add tests for multiple continuous states
• Adjust documentation

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.39%. Comparing base (7d7af99) to head (73645b0).

Files with missing lines	Patch %	Lines
src/dcegm/pre_processing/check_model_config.py	70.58%	5 Missing ⚠️
...rocessing/model_functions/upper_evelope_wrapper.py	57.14%	3 Missing ⚠️
src/dcegm/interfaces/model_class.py	66.66%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (66.66%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
- Coverage   92.73%   92.39%   -0.34%     
==========================================
  Files          68       68              
  Lines        2216     2236      +20     
==========================================
+ Hits         2055     2066      +11     
- Misses        161      170       +9     

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