Minimum/Maximum

[valentinsulzer]

Add new binary operators that return the elementwise minimum / maximum values (e.g. pybamm.Minimum([1,2,3], 2) = [2,2,3]. Two options:

1. Evaluate can be handled by np.minimum / np.maximum, and the derivative should be e.g. left.diff(var) * (left > right) + right.diff(var) * (left <= right) (easier but gives discontinuity of derivative at the switching point)
2. Implement some kind of smooth transition between the two, e.g. based on this