Refactor/put start and end on

[GoThrones]

Overview: What does this pull request change?

This PR refactors the method "put_start_and_end_on" in mobject.py and opengl_mobject.py:

Replaces self.points = np.array(start) with self.shift(np.asarray(start) - current_start) in the case of a mobject whose start and end point are the same, in both mobject.py and opengl_mobject.py

Adds a warning that warns the user that the mobject which is calling this method is a loop, and it will be shifted.

Some variable names have been changed for better reading.

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

The put_start_and_end_on method in both mobject.py and opengl_mobject.py previously used self.points = np.array(start) when the mobject had its start and end as the same point. This collapsed all of the mobject's points into a single Point3D, destroying its geometry entirely. For example, calling this method on a Circle or Square would reduce it to a single point, making it cease to exist visually.
This PR fixes the issue by replacing that line with self.shift(np.asarray(start) - current_start), which simply repositions the mobject to the start point while fully preserving its geometry.

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

[chopan050]

Thanks for your contribution! Shifting the Mobject in this case makes sense, and I appreciate both expanding the variable names and making the behavior consistent in Mobject and OpenGLMobject.

I left some change requests:

[chopan050]

As this old TODO mentioned, tests break when modifying this. In particular, there is a failing test in tests/module/mobject/graphing/test_number_line.py which must be fixed in order to approve and merge this PR:

def test_start_and_end_at_same_point():
    line = DashedLine(np.zeros(3), np.zeros(3))
    line.put_start_and_end_on(np.zeros(3), np.array([0, 0, 0]))

    np.testing.assert_array_equal(np.round(np.zeros(3), 4), np.round(line.points, 4))

Modifying the last assertion to

    np.testing.assert_array_equal(np.round(line.points, 4), np.round(np.zeros((4, 3)), 4))

should do the trick. Notice that:

• I flipped the argument order, because the 1st one should be the actual value and the 2nd one should be the desired value.
• The desired array is now np.zeros((4, 3)) instead of np.zeros(3).

[chopan050]

I believe that it is not necessary to leave a comment on how the old TODO was fixed:

Suggested change

	# Previously self.points = np.array(start) was used here, but it would have collapsed
	# all points to a single Point3D instead of shifting the mobject.
	# Fixed by using shift instead.

[chopan050]

Suggested change

	# Previously self.points = np.array(start) was used here, but it would have collapsed
	# all points to a single Point3D instead of shifting the mobject.
	# Fixed by using shift instead.