Changes to pysb.bng.run_ssa to make it more similar to
pysb.integrate.odesolve: 1) Changed the function signature to take a
list of output times (tspan) rather than t_end and n_steps (this allows
t_start to be non-zero). Also added an **additional_args kwarg so that
all other arguments to BNG's simulate_ssa() method can be used. 2) Added
the species populations from the CDAT file to the return array with
names '__s0', '__s1', etc.