Add the possibility to set the alpha channel in the meshcat viewer

[GiulioRomualdi]

If you want to use the idyntree-model-view-meshcat script with the alpha channel you can open a terminal and write

idyntree-model-view-meshcat -m <model_path/model.urdf> -a 0.5

This will show the model with an alpha equal to 0.5 as follows

In general, you can now load the urdf as

model_path = "model.urdf"
viz.load_model_from_file(model_path)  # in this case the color is retrieved from the urdf.
viz.load_model_from_file(model_path, color=0.5) # in this case the color is retrieved from the urdf but the alpha is set to 0.5 .
viz.load_model_from_file(model_path, color=[1,0,0,0.5]) # in this case the color is set to be [1,0,0] (RGB format) and the  alpha is set to 0.5.
viz.load_model_from_file(model_path, color=[1,0,0]) # in this case the color is set to be [1,0,0] (RGB format) and the  alpha is set to 1 (no transparency).

cc @rob-mau @Giulero

[traversaro]

Failures seem unrelated, @GiulioRomualdi can you add a note on the changelog?

[GiulioRomualdi]

Hi @traversaro, done in e43366f

[traversaro]

Can you move it under "Unreleased" ? We already released 7.0.0

[GiulioRomualdi]

Ops, my fault 😢 . Done in 95d9555

[traversaro]

Thanks!