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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.41%. Comparing base (acf1a88) to head (2c55d85).
⚠️ Report is 24 commits behind head on multi_cont.

Additional details and impacted files

@@             Coverage Diff             @@
##           multi_cont     #197   +/-   ##
===========================================
  Coverage       92.41%   92.41%           
===========================================
  Files              70       70           
  Lines            2413     2413           
===========================================
  Hits             2230     2230           
  Misses            183      183           

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[amageh]

All relevant changes are now covered in #198, so I am closing this PR.