Casadi solver doesn't work with external variables

Summary ODE solver allows you to specify external variables - it would be good if Casadi solver did to

At the very least there should be some error messages saying it’s not implemented yet. If you try to run it you get runtime errors

RuntimeError: …/casadi/core/function_internal.hpp:1247: Input 1 (i1) has mismatching shape. Got 80-by-1. Allowed dimensions, in general, are:

• The input dimension N-by-M (here 81-by-1)
• A scalar, i.e. 1-by-1
• M-by-N if N=1 or M=1 (i.e. a transposed vector)
• N-by-M1 if K*M1=M for some K (argument repeated horizontally)
• N-by-P*M, indicating evaluation with multiple arguments (P must be a multiple of 1 for consistency with previous inputs)

This was produced with the following script

import pybamm
import numpy as np
model_options = {
    "thermal": "x-lumped",
    "external submodels": ["thermal"],
}
model = pybamm.lithium_ion.SPMe(model_options)
solver = pybamm.CasadiSolver()
sim = pybamm.Simulation(model, solver=solver)
t_eval = np.linspace(0, 0.01, 3)
T_av = 0.0
for i in np.arange(1, len(t_eval) - 1):
    dt = t_eval[i + 1] - t_eval[i]
    external_variables = {"X-averaged cell temperature": T_av}
    T_av += 1.0
    sim.step(dt, external_variables=external_variables)

[TomTranter]

Fine by me . I approved the PR

[tinosulzer]

Let’s merge this in now, and I’ll work on other issues as part of #784