Fix singularities in atan2 at 0, 0 in swerve physics

The derivative of atan2 is
   -y / (x^2 + y^2), x / (x^2 + y^2)

This explodes at 0, 0.  Be much more gentle at 0, 0 in our calculation
to reduce the rate of change.  This makes the solver converge
*significantly* faster at 0, 0.  We still can't go sideways yet from 0,
0.

Change-Id: If5496714ec1fd36fac76913d208e803257dcd02f
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/control_loops/swerve/BUILD b/frc971/control_loops/swerve/BUILD
index fcd60a1..670bcc4 100644
--- a/frc971/control_loops/swerve/BUILD
+++ b/frc971/control_loops/swerve/BUILD
@@ -208,6 +208,7 @@
         ":physics_test_utils",
         "@pip//casadi",
         "@pip//numpy",
+        "@pip//scipy",
     ],
 )