Add the possibility to handle assemblies that have parts with the same name

[Nicogene]

Right now we are using the part name as linkname basically, and if an assembly contains 2 parts with the same name the urdf produced is invalid since it is not possible to have two links with the same name.

We should check if already exist the same name, print a warning and change the name adding a suffix as the simscape multibody link does

cc @salvi-mattia