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 ergodic distribution with k states is such that one of the states has a 0 probability.

This pr handles the divide by zero case.