Stability measures

[bcdaniels]

It would be nice to calculate some measures of stability.

I think a simple version would be based on the sensitivity measures we have already implemented. E.g. I think that's the point of this paper:
Activities and sensitivities in Boolean network models https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.93.048701

And a more fancy version is defined here:
The effect of network topology on the stability of discrete state models of genetic control
http://www.pnas.org/content/106/20/8209.full

[bcdaniels]

Addressed by #71