[KDL] Added some introspection to Path_Composite and Path_RoundedComposite

For some applications it is useful to have knowledge about where the
segments of a composite are located (in terms of the path variable s)
and which segment corresponds to a given path variable s.
Additional introspection methods are provided for

There is one caveat:
the path_roundedcomposite splits up its path in line semgents and arc
segments. This means that it adds additional segments.

To complicate things, when added points lie on a straight line, there
is no additional segment added ( because in that case, there is no
circular arc needed or even possible ).

One can keep track of this by observing getNrOfSegments() while adding
the points to Path_RoundedComposite.

An example use case for this routines, is when you want to interpolate
an additional variable ( e.g. impedance) along a trajectory.

Changes are committed to

Best regards,