Add increment using += support for ValueTracker

Author: nilaybhatia

manim/mobject/value_tracker.py

@@ -57,6 +57,9 @@ def set_value(self, value):
     def increment_value(self, d_value):
         self.set_value(self.get_value() + d_value)
 
+    def __iadd__(self, d_value):
+        return ValueTracker(self.get_value() + d_value)
+
     def interpolate(self, mobject1, mobject2, alpha, path_func=straight_path):
         """
         Turns self into an interpolation between mobject1