minimize should provide a consistent interface

[James Maddison]

Currently the arguments accepted by the minimize function vary depending upon the optimisation algorithm selected, as arguments are passed directly to the relevant optimisation function. This makes it difficult to change between optimisation algorithms, and requires reference to the documentation of lower level libraries.

[Simon Funke (simon-funke)]

I just saw that the new scipy.optimize version offers a unified interface to its algorithms, which we can use.

In the long term (in particular if dolfin-adjoint connects to mutliple optimisation frameworks) I will introduce a mapping dictionary that provides a unified interface - at least as much as possible.