Write tutorial on loading iDynTree::Model from URDF, do some modification, and write it back to URDF

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There is a lot of interest among several groups on this idea of programmatically modify or create URDFs models (tipically in the context of co-design or similar) so it could make sense to write a tutorial to showcase the iDynTree available features in this context.

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fyi @CarlottaSartore @VenusPasandi @fabiodinatale @valentino-razza @Nicogene @GrmanRodriguez