When adding two spatial inertia using the + override I get a AttributeError: 'SpatialInertia' object has no attribute 'I'.

I think I'm noticing 3 potential mistakes in the source code at line 637

return SpatialInertia(left.I + left.I)

1. It seems SpatialInertia has no member I but rather a data[0]
2. We're adding left twice and not right.
3. I am not sure the syntax matches the definition in the constructor...

To make minimum changes, I would suggest rewritting this line as such:

total_inertia=SpatialInertia()
total_inertia.data[0] = left.data[0] + right.data[0]
return total_inertia

But I'm not familiar with the whole code corpus, and I just infer that from quickly reading the class. Does that seem correct? If so could we fix it? If it helps I am happy making a PR and expending tests so this doesn't happen anymore.

Otherwise, thanks for a cool library!