Improve the optimisation documentation

[Patrick Farrell]

Two improvements:

a) The names of the algorithms should use the real scipy names. I read about scipy.slsqp, and wanted to look up the documentation for it (since I need it for the keywords), but it took me a few minutes to realise it's actually scipy.optimize.slsqp.

b) You should explain how to add callbacks in the optimisation documentation. The word "callback" doesn't appear on http://dolfin-adjoint.org/documentation/optimisation.html .

[Simon Funke (simon-funke)]

Both of these are now implemented.
Callbacks are available if the user has scipy >= 0.11