Add methods to access underlying model, problem to errors/problems that have them

[MichaelClerx]

E.g.

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

Not needed from point of view of evaluating / simulating, but very convenient when e.g. passing a problem to a plotting method if it can also get parameter names from underlying model etc