Allow drive cycles in experiments

Add functionality to allow running a drive cycle as part of an experiment. For example:

pybamm.Experiment(["Run US06 for 1 hour"], functions={"US06": US06_function})

Currently, the way experiments are implemented require current/voltage/power to be constant (they are set up as InputParameter objects). So this could be challenging to implement, and would depend on #895 being possible.

[tinosulzer]

Closed via #1793

[Saransh-cpp]

With the new setup type the Current input (and actually voltage input and power input) are only needed for the parameter values. So once you update the parameter values you can throw them away. Something like

I = op_inputs.pop("Current input [A]")
new_parameter_values.update("Current function [A]": I)

might just work out of the box. Since op_inputs is a pointer to an object in self._experiment_inputs this will also remove “Current input [A]” from the relevant dictionary in self._experiment_inputs. operating_inputs should be unchanged

I will definitely look into this and if everything goes fine, I will make a PR.