Fix return type of Polygram.get_vertex_groups() and rename variables in .round_corners()

[chopan050]

Polygram.get_vertex_groups() returns a sequence of point arrays representing the vertex groups of Polygram. Currently, it tries to return a single 3D NumPy array, but that assumes that all the vertex groups have the same length, which does not always happen. When it doesn't happen, this function crashes.

Instead, this PR modifies this method to return a list[Point3D_Array]: a list of 2D NumPy arrays.

Plus, this PR renames variables in Polygram.round_corners():

• vertices -> vertex_group
• currentRadius -> current_radius
• nonZeroLengthArcs -> nonzero_length_arcs
• totalCurveCount -> num_curves
• averageLengthPerCurve -> average_arc_length

Links to added or changed documentation pages

https://manimce--4063.org.readthedocs.build/en/4063/reference/manim.mobject.geometry.polygram.Polygram.html#manim.mobject.geometry.polygram.Polygram.get_vertex_groups

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