Update opacity management in actor.sphere

[skoudoro]

After looking more in detail into your fix, I think we should avoid using actor.GetProperty().SetOpacity(opacity). It seems not compatible with our architecture.
Ideally, in the function actor.sphere, we would check if opacity != 1. if yes, we should update the color array by adding a dimension if it is an Nx3, or by updating the last dimension if it is an Nx4.

I will merge your PR because it fixes the actor.point but we need a new PR to improve this version.

Originally posted by @skoudoro in #351 (comment)

[tantheta01]

I have generated a PR for this issue. Thank you.