Fix link filtering

[Giulero]

This PR fixes #82.
The link filtering was done by selecting the links with inertia (and frames without inertia; see #83 (comment)).

With this fix, massless links with children are also considered connecting links, and the tree is not broken.

[Giulero]

The parametric interface is failing. I'm gonna fix it

[Giulero]

Parametric interface fixed. I've also added the additional check #83 (comment)

Namely, citing idyntree:

/**
 * The condition for a link to be classified as "fake link" are:
 *  * The link has a zero mass.
 *  * The link is a leaf, i.e. it is connected to only one neighbor.
 *  * The link is connected to its only neighbor with a fixed joint.
 */

[Giulero]

Thanks guys! :) Merging.