Adding methods to allow access to problems, models, etc underlying errors, priors, etc

[MichaelClerx]

See #1706

• ProblemErrorMeasure.problem()
• ProblemLogLikelihood.problem()
• SingleOutputProblem.model()
• MultiOutputProblem.model()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e8dd222) to head (96fe8fd).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1715   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           97        97           
  Lines         9693      9701    +8     
=========================================
+ Hits          9693      9701    +8     

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