Modified pysb.bng.run_ssa to include 'time' in the output array. This is
important because it's possible for the ouput times from BNG to be
different from the time points specified in the input 'tspan' array. For
example, the 'output_step_interval' argument tells BNG to output at
specified numbers of simulation steps. Since BNG will also output at the
specified time intervals, the number of output time points will exceed
the number of input time points. Another example is the 'stop_if'
argument, which tells BNG to quit if a user-specified logical condition
is met, e.g., a species concentration exceeds a threshold. In this case,
the number of output time points will be less than the number of input
time points.