v2: Observable/conditions/experiments/... objects

[dweindl]

Implement an object model for the various PEtab entities as an alternative to working with the plain DataFrames.

Related to #337.

Object names are very likely to change. This is more of a proof of concept.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 79.68254% with 64 lines in your changes missing coverage. Please review.

Project coverage is 75.14%. Comparing base (ec58463) to head (fca15e4).

Files with missing lines	Patch %	Lines
petab/v2/core.py	79.01%	47 Missing and 17 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #345      +/-   ##
===========================================
+ Coverage    74.89%   75.14%   +0.25%     
===========================================
  Files           56       57       +1     
  Lines         5604     5919     +315     
  Branches       982     1018      +36     
===========================================
+ Hits          4197     4448     +251     
- Misses        1033     1080      +47     
- Partials       374      391      +17     

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