Return the distribution of SSB at zero F

[colinpmillar]

In some cases, either productivity is too low, or B_limis too high and the equilibrium stock size distribution is such that B_lim can not be reached and exceeded with enough frequency in the simulations to permit estimation of F_lim (Prob(SSB > B_lim) = 0.5) or F_p05 (Prob(SSB > B_lim) = 0.95).

It would be useful to see where B_lim sits in terms of the equilibrium population size

[colinpmillar]

in part linked to
Add feature to return simulation time series #19