Repository: ManimCommunity/manim
Pull request: Fixed "get_arc_center()" method of Arc class
## Explaination
`get_arc_center()` method fails if the arc radius is zero. Therefore an if statement has been added to indirectly check whether the arc radius is zero or not.
If the anchor points lie at the same point, it implies that arc radius is zero. Thus arc_center also lie at that point.

This PR fixes #480 

## Acknowledgement
- [x] I have read the [Contributing Guidelines](https://github.com/ManimCommunity/manim/wiki/Documentation-guidelines-(WIP))


Problem statement:
Sector.get_arc_center() is broken.
Sector.get_arc_center() does not work and always returns a warning message.
```sh
UserWarning: Can't find Arc center, using ORIGIN instead
 warnings.warn("Can't find Arc center, using ORIGIN instead")
```
Above error can be reproduced using the below code.
```py
from manim import *
class Test(Scene):
    def construct(self):
        sec = Sector(start_angle=0, angle=5*PI/3).move_to([1,0,0])
        print(sec.get_arc_center())
```

Hints from issue discussion:
Thanks for the report. Something tells me that this *might* even be the desired behavior, but the way it is handled right now, it's impossible to be certain. Cc @ManimCommunity/geometry any takers?
@leotrs  I have solved this issue and I'll soon send a PR

Commit message: Added test case for get_arc_center()

Patch to review:
diff --git a/manim/mobject/geometry.py b/manim/mobject/geometry.py
index 74ab65e9ed..f234ff432a 100644
--- a/manim/mobject/geometry.py
+++ b/manim/mobject/geometry.py
@@ -288,6 +288,12 @@ def get_arc_center(self, warning=True):
         """
         # First two anchors and handles
         a1, h1, h2, a2 = self.points[:4]
+
+        if np.all(a1 == a2):
+            # For a1 and a2 to lie at the same point arc radius
+            # must be zero. Thus arc_center will also lie at
+            # that point.
+            return a1
         # Tangent vectors
         t1 = h1 - a1
         t2 = h2 - a2
diff --git a/tests/test_geometry.py b/tests/test_geometry.py
new file mode 100644
index 0000000000..f3c853a2b6
--- /dev/null
+++ b/tests/test_geometry.py
@@ -0,0 +1,7 @@
+import pytest
+import numpy as np
+from manim import Sector, ORIGIN
+
+
+def test_get_arc_center():
+    assert np.all(Sector().get_arc_center() == ORIGIN)


Code review comments:
diff_hunk: @@ -0,0 +1,7 @@
+import pytest
path: tests/test_geometry.py
text: @user1:
Unfortunatly there is already a test_geometry file, which is for graphical unit tests of geometrey module. Could you please rename this file into something like "test_unit_geometry.py'" ? 

@author:
Done