Add a GITT fitting example

[NicolaCourtier]

Feature description

Create an example workflow for fitting GITT test data.

Motivation

To demonstrate and benchmark a common battery parameterisation problem.

Possible implementation

This could be implemented as an example script/notebook or as an extension of the FittingProblem class.

Additional context

No response

[brosaplanella]

We did something similar in pybamm-param (see https://github.com/paramm-team/pybamm-param/blob/develop/pbparam/optimisation_problems/gitt.py). Happy to give it a go, as this would inform how to integrate both projects.