Readding the constants TOP, BOTTOM, LEFT_SIDE and RIGHT_SIDE

[kolibril13]

In constans.py,
there once where the constants:

TOP = FRAME_Y_RADIUS * UP
BOTTOM = FRAME_Y_RADIUS * DOWN
LEFT_SIDE = FRAME_X_RADIUS * LEFT
RIGHT_SIDE = FRAME_X_RADIUS * RIGHT

I find them very useful, so I think we should include them back there.

When I am in a scene at the moment, my workaround is

        TOP = Camera().get_frame_height() / 2 * UP
        BOTTOM = Camera().get_frame_height() / 2 * DOWN
        LEFT_SIDE = Camera().get_frame_width() / 2 * LEFT
        RIGHT_SIDE = Camera().get_frame_width() / 2 * RIGHT

[kolibril13]

I just saw the conversation in #283 ,
and I also like the idea of

frame.top instead of the clunkier and less-readable camera_config["top"]

But as there is LEFT, UP and RIGHT, as a user I would expect that there would be as well e.g. TOP and BOTTOM.

[leotrs]

Yes this is a tricky one. They had to be removed from constants.py because they aren't constants. But we don't want to get in the habit of populating the global manim namespace even more than it is now, especially not from the config.py file.

So my workaround is that all of those go into a new frame, but I don't think we have reached consensus there.

I guess we can put TOP and the others back in the manim namespace for the time being, for backwards compatibility.