draft of section group feature

[barollet]

Overview:

Adds a group decorator to Scene that are basically "sections group".

class SceneWithGroupAPI(Scene):
    groups_api = True # groups are read from top to bottom

    def __init__(self):
        super().__init__()

        self.square = Square()
        self.circle = Circle()

    @group
    def transform(self):
        self.play(TransformFromCopy(self.square, self.circle))

    @group
    def back_transform(self):
        self.play(Transform(self.circle, self.square))


class SceneWithGroupList(Scene):
    section_groups = ["transform", "back_transform"] # group order is specified by user (groups may even be repeated)

    def __init__(self):
        super().__init__()

        self.square = Square()
        self.circle = Circle()

    @group
    def back_transform(self):
        self.play(Transform(self.circle, self.square))

    @group
    def transform(self):
        self.play(TransformFromCopy(self.square, self.circle))

Motivation and Explanation: Why and how do your changes improve the library?

For long videos you may want to have 2 levels of granularity for skipping/animation parts of the videos. Sections are the basic building block like a few animations and they are rendered in the same way as before.
You may add groups that behave like "chapters". They are a sequence of sections (you can still put sections inside groups) and the skipping/animation of section groups takes precedence over the individual section skipping/animation.

To make the distinction between sections and section groups the experimental API for sections is used. This way creating section is unchanged and we can gain the flexibility of the experimental API. Actually most of the code for the definition of a SectionGroup is taken from the experimental Section object.

Links to added or changed documentation pages

The documentation must be done but the API is not stabilized. I'll wait for the namming to be correct before documenting.
It should go in the section page I think.

Additional comments

There is a feature in the SectionGroup object to make it become a full member of the Scene instance that I couldn't make work.
For now section groups must be called like group(self).

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested