[Bug]: Error if variable bounds contain an InputParameter

[rtimms]

Now that we switched to using dimensional variables, if you have an InputParameter in the bounds of a variable you can't discretise it. We should skip the bounds in this case IMO, otherwise, you would need to re-discretize each time you changed the input.

Code to reproduce this:

import pybamm
model = pybamm.lithium_ion.SPM()
param = pybamm.ParameterValues("Chen2020")
param["Maximum concentration in positive electrode [mol.m-3]"] = "[input]"
sim = pybamm.Simulation(model, parameter_values=param)
sim.solve([0, 3600], inputs = {"Maximum concentration in positive electrode [mol.m-3]": 65000})