ENH: handle the case of an ergodic distribution where one state has 0 probability

This is in reference to https://groups.google.com/forum/#!topic/openspace-list/wj0A3_VhLrU

Numpy would issue a warning of divide by zero. This occurs in cases where the chain is irregular resulting in the  estimated "ergodic" distribution with k states is such that one or more of the states has a 0 probability. Since the chain is irregular it does not have an ergodic distribution.

This pr handles the divide by zero case.