DoubleArrow Incorrect behaviour in 3D

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## Description of bug / unexpected behavior

A double arrow in the z axis generates a tip pointing in an orthogonal direction

## Expected behavior

Both tips of the arrow should be collinear.

## How to reproduce the issue

Code for reproducing the problem

```py
class TestArrows3D(ThreeDScene):
def construct(self):
self.set_camera_orientation(phi = 70 * DEGREES, theta = -45 * DEGREES,
zoom = 2)
h = 0.5 + 0.1
Lx = DoubleArrow([-0.5,- h ,- 0.5], [0.5,- h ,- 0.5])
Ly = DoubleArrow([h ,- 0.5,- 0.5], [h , 0.5,- 0.5])
# The next double arrow has a wrong tip
Lz = DoubleArrow([0.5 ,h ,- 0.5], [0.5, h , 0.5], color = RED)
self.add(Cube(1), Lx, Ly, Lz)
```

## Additional media files

Images/GIFs


## Logs
Terminal output

```
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```

## System specifications

System Details

- OS (with version, e.g Windows 10 v2004 or macOS 10.15 (Catalina)):
- RAM:
- Python version (`python/py/python3 --version`):
- Installed modules (provide output from `pip list`):
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LaTeX details

+ LaTeX distribution (e.g. TeX Live 2020):
+ Installed LaTeX packages:

FFMPEG

Output of `ffmpeg -version`:

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```

## Additional comments

Start with the provided TestArrows3D.construct reproduction and inspect the DoubleArrow implementation used by the z-axis case. Compare the generated tips against the x- and y-axis cases, then add or update a regression test if the repository's test location is identified. Done means both tips of a z-axis DoubleArrow are collinear and the reproduction passes.

Written by the indexing model from the issue text.