Smooth rotations: Use rate_func for CubeMove

[vvolhejn]

This change makes CubeMove use the rate_func parameter that specifies how the animation progresses. So by default, the rotation will be smooth, whereas the current default is a linear function (constant speed). To revert back to the old behavior, rate_func=linear can be passed to Scene.play, e.g.

self.play(CubeMove(cube, "F"), run_time=1, rate_func=linear)

[WampyCakes]

I'm not really ready to merge either of your PRs as I have a lot of changes locally that I think should be incorporated first. I think you have made some good suggestions and improvements, and I will look to incorporate some of them in the future (depending on how much you care about contributions on GH, either directly or through an edited PR by you). I am currently unable to give an estimate of when I'll do this. In the meantime, you're getting a nice local edit of manim-rubikscube going that should serve you well!