Feature to break the simulation when the steady state has been reached

[Adrian Pop]

Proposed here:
​http://stackoverflow.com/questions/42673420/global-measure-to-test-when-steady-state-of-system-is-reached

The first point sounds like a good to have feature and doesn't seem that hard to implement, basically a terminate when all the ders are 0 (within an epsilon) for a number of steps.

For the second point we would need a global operator that returns i.e. the sum of all ders that one can use in equations to enable / disable parts of the system.