[SymForce-External] Add to_angle for the Rot2 class

Add `to_angle` for the `Rot2` class

I suggest adding a convenience function to get the angle from a 2D rotation. It complements the existing `from_angle`, and is intuitive to use when working with 2D rotation angles.

Closes #358

GitOrigin-RevId: 04bfe2a4ac1e658b218287e96cd1d2f7505dedf1

symforce/geo/rot2.py

@@ -137,6 +137,14 @@ def from_angle(cls, theta: T.Scalar) -> Rot2:
         """
         return cls.from_tangent([theta])
 
+    def to_angle(self, epsilon: T.Scalar = sf.epsilon()) -> T.Scalar:
+        """
+        Get the angle of this Rot2 in radians
+
+        This is equivalent to ``to_tangent()[0]``
+        """
+        return self.to_tangent(epsilon)[0]
+
     def to_rotation_matrix(self) -> Matrix22:
         """
         A matrix representation of this element in the Euclidean space that contains it.

test/geo_rot2_test.py

@@ -49,6 +49,19 @@ def test_angle_constructor(self) -> None:
         rot2 = sf.Rot2.from_tangent([1.5])
         self.assertEqual(rot1, rot2)
 
+    def test_from_to_angle(self) -> None:
+        """
+        Tests:
+            Rot2.from_angle
+            Rot2.to_angle
+        """
+        for angle, angle_gt in zip(
+            [0.0, np.pi / 2, np.pi, 3 * np.pi / 2, 2 * np.pi],
+            [0.0, np.pi / 2, np.pi, -np.pi / 2, 0.0],
+        ):
+            rot = sf.Rot2.from_angle(angle).evalf()
+            self.assertLess(abs(angle_gt - rot.to_angle()), 1e-8)
+
     def test_lie_exponential(self) -> None:
         """
         Tests: