Raise warning when converting pchip interpolants to casadi

[rtimms]

When a pybamm.Interpolant object is created you can set the interpolator to "linear", "pchip" or "cubic spline" (default). When this node gets converted to casadi, the converted interpolator is always a spline. We should raise a warning if an Interpolant using "linear" or "pchip" gets converted to casadi's interpolant so that users know it is switching to a spline.