Python interpreter loop brutally adjusts time step, methods may not allow that

Here:

https://gitlab.tiker.net/inducer/leap/blob/master/leap/vm/exec_numpy.py#L131

For example, AB methods will be quite unhappy. This means the only semantics we can support is stepping past the desired end time step. I'm not sure that's entirely satisfactory, but I'm also not sure we should introduce a "time step change" interface and force every method to support that...