Refactor many definitions out of topics and into folders within mobject

mobject/value_tracker.py

@@ -0,0 +1,43 @@
+from __future__ import absolute_import
+
+import numpy as np
+
+from constants import *
+
+from mobject.types.vectorized_mobject import VectorizedPoint
+
+# TODO: Rather than using VectorizedPoint, there should be some UndisplayedPointSet type
+
+class ValueTracker(VectorizedPoint):
+    """
+    Note meant to be displayed.  Instead the position encodes some 
+    number, often one which another animation or continual_animation
+    uses for its update function, and by treating it as a mobject it can
+    still be animated and manipulated just like anything else.
+    """
+    def __init__(self, value = 0, **kwargs):
+        VectorizedPoint.__init__(self, **kwargs)
+        self.set_value(value)
+
+    def get_value(self):
+        return self.get_center()[0]
+
+    def set_value(self, value):
+        self.move_to(value*RIGHT)
+        return self
+
+    def increment_value(self, d_value):
+        self.set_value(self.get_value() + d_value)
+
+class ExponentialValueTracker(ValueTracker):
+    """
+    Operates just like ValueTracker, except it encodes the value as the
+    exponential of a position coordinate, which changes how interpolation 
+    behaves
+    """
+    def get_value(self):
+        return np.exp(self.get_center()[0])
+
+    def set_value(self, value):
+        self.move_to(np.log(value)*RIGHT)
+        return self