Introduce intermediate variables for selected nodes in a graph.

::

 extract([DM] ex, dict opts) -> ([DM] OUTPUT1, [DM] OUTPUT2, [DM] OUTPUT3)
 extract([SX] ex, dict opts) -> ([SX] OUTPUT1, [SX] OUTPUT2, [SX] OUTPUT3)
 extract([MX] ex, dict opts) -> ([MX] OUTPUT1, [MX] OUTPUT2, [MX] OUTPUT3)

Extra doc: https://github.com/casadi/casadi/wiki/L_1d5

Doc source: https://github.com/casadi/casadi/blob/develop/casadi/core/generic_matrix.hpp#L959

Implementation: https://github.com/casadi/casadi/blob/develop/casadi/core/generic_matrix.hpp#L959-L964

To edit, see writing tips.