pouch cell initial conditions

[rtimms]

The potential_pair model sets the initial conditions uses applied_current = variables["Total current density"]. For non-constant current discharge this is part of the statevector and leads to a discretisation error (the initial conditions aren't a numpy array). For now, his should be replaced with applied_current = self.param.current_with_time.

Question: are there any cases where it would be convenient to set ICs in terms of another variable (provided ICs set for that variable evaluate to a numpy array)?