Reset the stepsize to initial value at the beginning of each pulse

[RemDelaporteMathurin]

For stability purposes, it would be preferable to reset the value of the stepsize to its initial value at the beginning of each pulse.

We can do this in here by adding a new milestone just after the beginning of the pulse. Say the pulse starts at t=200s and we want the stepsize to be 1 second, then we set another milestone at t=201s

https://github.com/kaelyndunnell/hisp/blob/d785b7d196150a003472f31d9ee111596b69a9ea/src/hisp/model.py#L165-L189

We may need an optional argument to this method.