Add idyntree-model-simplify-shapes command line tool

[traversaro]

A recurring pattern I noticed in people working in co-design/optimizing model parameters based on some task, is that tipically they want as a starting point something based on a real robot, but without the strange visual or collision solid shapes, because it is too difficult or complex to optimize mesh based shapes. For this reason, it is necessary to switch to a model similar to the starting one, but with all meshes substitued with a simple primitive shape.

At the moment, I am not aware of how this can be done automatically. As we already have a quite simple iDynTree::computeBoundingBoxFromShape, I guess it should be trivial (basically just a few for loops) to do a idyntree-model-simplify-shapes command line tool that could be used like:

idyntree-model-simplify-shapes -m <complex-model.urdf> -o <simple-model.urdf>

However, to ensure that this tool is easy to use we need to first make sure that assimp dependency is enabled by default in robotology-superbuild and conda-forge:

• Enable assimp dependency in iDynTree  robotology-superbuild#917
• Enable assimp dependency in iDynTree conda-forge/idyntree-feedstock#14

[traversaro]

fyi @CarlottaSartore @fabiodinatale @VenusPasandi @valentino-razza

[traversaro]

Note that in theory more complex "approximation strategy" could be used instead of just the simple axis-aligned bounding-box that is just the easier strategy, but as as initial prototype just using axis-aligned bounding-box is probably enough.