Fix numpy type bug

[Woz4tetra]

Fix for numpy 1.20

I was getting this exception:

AttributeError: module 'numpy' has no attribute 'float'.
`np.float` was a deprecated alias for the builtin `float`. To avoid this error in existing code, use `float` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.float64` here.
The aliases was originally deprecated in NumPy 1.20; for more details and guidance see the original release note at:
    https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations. Did you mean: 'cfloat'?

I've also added materials to the animation so the models aren't just grey.

[traversaro]

See https://github.com/mmatl/urdfpy/issues/31 .