Add an example that uses a pybamm.FunctionalParameter

[NicolaCourtier]

Feature description

Add an example that demonstrates how to employ a pybamm.FunctionalParameter to estimate:

• a constant parameter that is fixed by definition in PyBaMM, e.g. the reference exchange current density
• a functional parameter that is set as a constant parameter in PyBaMM, e.g. the contact resistance

Motivation

This would provide a starting point for #150, #346 and it would also be useful for #269.

Possible implementation

No response

Additional context

No response