quaternion: Added generalized construction from and conversion to Euler-angles

The particular rotation sequence is specified via the enumeration:

    //- Euler-angle rotation sequence
    enum rotationSequence
    {
        ZYX, ZYZ, ZXY, ZXZ, YXZ, YXY, YZX, YZY, XYZ, XYX, XZY, XZX
    };

and provided as an argument to the constructor from Euler-angles

    //- Construct a quaternion given the three Euler angles:
    inline quaternion
    (
        const rotationSequence rs,
        const vector& angles
    );

and conversion to Euler-angles:

    //- Return a vector of euler angles corresponding to the
    //  specified rotation sequence
    inline vector eulerAngles(const rotationSequence rs) const;