ODE solver fails if propensity_function contains "vol"

[briandrawert]

This works for the ODECSolver, but not the ODESolver.

This is from the "KineticModelForStyrenePolymerization.ipynb" example:

NameError: name 'vol' is not defined

During handling of the above exception, another exception occurred:

SimulationError                           Traceback (most recent call last)
<ipython-input-12-40050f13f8ae> in <module>
      1 from gillespy2 import ODESolver, ODECSolver
----> 2 r1 = model.run(solver=ODESolver)
      3 r2 = model.run(solver=ODECSolver)
      4 #r1.plot()
      5 r2.plot()

~/Desktop/research/GillesPy2/examples/AdvancedFeatures/../../gillespy2/core/model.py in run(self, solver, timeout, t, increment, show_labels, cpp_support, algorithm, **solver_args)
    997                                 " system, to ensure that GillesPy2 C solvers will"
    998                                 " run properly.")
--> 999             raise SimulationError(
   1000                 "argument 'solver={}' to run() failed.  Reason Given: {}".format(solver, e))
   1001 

SimulationError: argument 'solver=<class 'gillespy2.solvers.numpy.ode_solver.ODESolver'>' to run() failed.  Reason Given: name 'vol' is not defined