Added grid_lines attribute to Rectangle to add individual styling to the grid lines

[RobinPH]

Overview: What does this pull request change?

Added grid_lines (VGroup) attribute to Rectangle class. Both vertical, and horizontal grid lines are added to it (if there is any). It allows more styling capability to grid lines.

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

Adds more customizability for Rectangle class

Links to added or changed documentation pages

https://manimce--3428.org.readthedocs.build/en/3428/reference/manim.mobject.geometry.polygram.Rectangle.html

Further Information and Comments

Example code:

from manim import *


class RectangleGridLineStrokeExample(Scene):
    def construct(self):
        rect1 = Rectangle(width=4.0, height=3.0, grid_xstep=1.0, grid_ystep=1.5)
        rect2 = Rectangle(width=4.0, height=3.0, grid_xstep=1.0, grid_ystep=1.5)
        rect2.grid_lines.set_stroke(width=1)

        rects = VGroup(rect1, rect2).arrange(buff=1)

        self.add(rects)

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

[behackl]

Hey there! Thanks for your contribution!

I am somewhat concerned about the general approach here: while it is true that more control over the styling of the grid lines definitely makes sense, I am not sure whether adding more input arguments to the constructor of Rectangle is the best decision -- we might want similar control for, say, color and opacity too.

My counter-proposal for this would look as follows: we add a grid_lines attribute, populated by an empty VGroup,

self.grid_lines = VGroup()

and inside of the two line generating blocks, we add the generated line groups not as submobjects of the rectangle directly, but as submobjects of self.grid_lines:

self.grid_lines.add(grid)

And then, in the end, we can check whether or not the newly created group is empty -- and if it is not, then we can add the entire group to the rectangle:

        if self.grid_lines:
            self.add(self.grid_lines)

Then, the styling of the grid lines can be controlled easily via

rect1 = Rectangle(width=4.0, height=3.0, grid_xstep=1.0, grid_ystep=1.5)
rect1.grid_lines.set_stroke(width=1)

What do you think about this?

[MrDiver]

LGTM!

Changes were done