Merge Oracle and Function

[jaeandersson]

The Oracle class provides a useful abstraction for a "Function factory". There is no reason though why this functionality could not be moved into the Function class. This would improve maintainability and make the functionality of the now internal Oracle class available to the end user:

from casadi import *
# Create a function
x = SX.sym('x')
y = SX.sym('y')
f = Function('fcn', [x, y], [y*sin(x), x*cos(y)], ['x', 'y'], ['r', 'q'])
# Create a function for calculating r only
r = f.factory('rfcn', ['x', 'y'], ['r'])
# Create a function for calculating the Jacobian of q wrt y
dqdy = f.factory('dqdy', ['x', 'y'], ['jac_q_y'])