Upgrade to object-oriented ODE solver

The current ODE solver [odeint](http://docs.scipy.org/doc/scipy/reference/generated/scipy.integrate.odeint.html#scipy.integrate.odeint) is inflexible and suffers from problem of stiffness as explained [here](http://stackoverflow.com/questions/16973036/odd-scipy-ode-integration-error) in some cases.

It is better to upgrade to ODE solver object-oriented [ode](http://docs.scipy.org/doc/scipy/reference/generated/scipy.integrate.ode.html), as it is more flexible and allows to set various options.