Potential multiple DiscreteUpdates being called

[yangwill]

As noted in #345, using DeclarePerStepDiscreteUpdateEvent can potentially cause the discrete update events to be called multiple times per AdvanceTo, which can cause unintended behavior.

Making this issue to inform/remind you to consider switching to DeclareForcedDiscreteUpdateEvent in your leaf systems to ensure it's only being called once per lcmdrivenloop.