Add functions for setting series data types

[dweindl]

Well-defined datatypes will simplify working with those PEtab tables.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 19.29825% with 46 lines in your changes missing coverage. Please review.

Project coverage is 74.31%. Comparing base (8325b53) to head (a14e010).

Files	Patch %	Lines
petab/v2/normalize.py	18.75%	39 Missing ⚠️
petab/v2/problem.py	22.22%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #303      +/-   ##
===========================================
+ Coverage    72.80%   74.31%   +1.50%     
===========================================
  Files           51       52       +1     
  Lines         4840     4897      +57     
  Branches      1035     1050      +15     
===========================================
+ Hits          3524     3639     +115     
+ Misses        1018      960      -58     
  Partials       298      298              

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