Need to be able to add/remove state variables on the fly

[sherm1]

For contact models like bristle friction, we would like to be able to add state variables when needed and remove them afterwards, without having to throw out the old state variables and existing cache entries.

While you're fixing this, consider how to avoid integrating states that correspond to prescribed motion since those are actually explicit functions of time.

Should the state contain objects representing variable-size pools of on-the-fly state variables? Will that work for discrete ones also? Do corresponding cache entries need to be allocated and destroyed too?