Get joint information from element tree

[mfussi66]

This PR updates the plugin by simplifying the retrieval of joint information:

• The element tree is used to get the constraint type (therefore joint type)
• From each part we get the parent link and the constraint axis in case of Pin, and coordinate system in case of Fixed

[Nicogene]

LGTM! Maybe we can think of moving some functions that are not strictly related to the elementTree in Utils.h but this could be done in future refinement.

I would edit the README removing the assumptions that now we overcome with this new implementation!

[Nicogene]

Today I tested these changes on my laptop and:

• It compiles
• It works
• After exporting the ergoCub urdf the diff in ergocub-software is null, this means we managed to make it more robust keeping the functionalities.

Merging!