Add solow model

Placeholder PR for merging a continuous-time version of he  Solow growth model into `quantecon.models`. Basic primitives of the Solow model are an aggregate production function and dictionary of parameter values. I want the user to  provide a sympy expression for aggregate production and some parameters, and then the class methods should set of the resulting differential equation compute the jacobian and pass these to the `ivp.IVP` class.

As always feedback is much appreciated!