Allow constructing control loops from flatbuffers

The core changes here are to:
* Allow constructing StateFeedbackLoop's from flatbuffers using the
  code in *state_feedback_loop_converters.*
* Add constructors to the single-dof subsystem class to make use of
  this.
* Add code to control_loops.py to generate JSON files with the requisite
  constants (these end up containing identical information to the
  generated .cc files).
* Add interfaces to actually support the new JSON codegen to single-dof
  subsystem classes.
* Convert all of the drivetrains over to generating these. This I mostly
  do so that I can write a test where Iconfirm that the .cc files and
  the JSON files generate exactly the same content.

Change-Id: Iceac48f25ecac96200b7bf992c8f34a15fe6800c
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/frc971/control_loops/python/linear_system.py b/frc971/control_loops/python/linear_system.py
index 3bda4e1..e28c422 100755
--- a/frc971/control_loops/python/linear_system.py
+++ b/frc971/control_loops/python/linear_system.py
@@ -422,8 +422,11 @@
                               linear_systems[0].G * linear_systems[0].radius))
     loop_writer.AddConstant(
         control_loop.Constant('kRadius', '%f', linear_systems[0].radius))
-    loop_writer.Write(plant_files[0], plant_files[1])
+    loop_writer.Write(plant_files[0], plant_files[1],
+                      None if len(plant_files) < 3 else plant_files[2])
 
     integral_loop_writer = control_loop.ControlLoopWriter(
         'Integral' + name, integral_linear_systems, namespaces=year_namespaces)
-    integral_loop_writer.Write(controller_files[0], controller_files[1])
+    integral_loop_writer.Write(
+        controller_files[0], controller_files[1],
+        None if len(controller_files) < 3 else controller_files[2])