Allow functions of time in experiment step

[valentinsulzer]

Users should be able to pass in arbitrary functions of time as the argument for a pybamm.step in an experiment, e.g.

def oscillating(t):
    return 1 + 0.1*np.sin(2*np.pi*t)

pybamm.step.current(oscillating, duration=3600)

Time should be taken to start at the beginning of the step, just like with drive cycles.

Note: this can be already be done using custom steps, but it requires writing a more complex step object