Allow the state y to be an array of arbitrary rank

[mattja]

Although scipy.integrate.odeint does not support that sort of thing.
But hey, why not?

This would then be an improved fix for mattja/nsim#2 , since it's reasonable for people to
supply a state of shape (n,1) interpreted as a column vector and expect it to work.

Implement with numpy.ravel on input and numpy.reshape on output but make sure data is not copied during reshape.