Add support for complex noise formulae in roadrunner

[PaulJonasJost]

This currently assumes two things:

• The noise formula for one observable is always the same
• There is no time dependent changes in the parameters of the formula

I think both assumptions are things we make implicitly in PEtab/pyPESTO?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.39%. Comparing base (45edfd2) to head (761eb3e).

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1363      +/-   ##
===========================================
- Coverage    84.44%   84.39%   -0.06%     
===========================================
  Files          157      157              
  Lines        12899    12914      +15     
===========================================
+ Hits         10893    10899       +6     
- Misses        2006     2015       +9     

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[Doresic]

Looks good!

[dweindl]

• There is no time dependent changes in the parameters of the formula

I think both assumptions are things we make implicitly in PEtab/pyPESTO?

I don't think pypesto should assume that without verification.
For PEtab/amici it's just a limitation of amici.