Storing external forces and contacts

[Samir Menon]

Should have a neat dynamics interface for this:

/** The external forces applied on the robot:
     * Eg.
     * a) By a user interacting through a gui
     * b) By a physical force on a real robot
     *
     * NOTE TODO : Consider using a CMappedList here
     * instead of the vector. The vector doesn't support
     * removing contacts very efficiently if there are a
     * large number of contacts. CMappedList is much
     * faster at removal/insertion. */
    std::vector<SForce*> forces_external_;