This PR adds the get link transformation method in the serial manipulator classes

[ffasilva]

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Description of changes

This PR adds the abstract method get_link_transformation() to the DQ_SerialManipulator class and provides concrete implementations of it in the DQ_SerialManipulatorDH and DQ_SerialManipulatorMDH classes.

This method exposes the get_link2dq() methods of the concrete classes, which calculate the corresponding dual quaternion for a given link's parameters.

Rationale

This is one of the methods proposed in the discussions regarding the DQ_Dynamic class, and is required to simplify how the DQ_DynamicsSolver calculates relative displacements between links.