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/drivetrain.py b/frc971/control_loops/python/drivetrain.py
index 5ff4404..65b30c0 100644
--- a/frc971/control_loops/python/drivetrain.py
+++ b/frc971/control_loops/python/drivetrain.py
@@ -525,7 +525,8 @@
             "kHighOutputRatio", "%f",
             drivetrain_high_high.G_high * drivetrain_high_high.r))
 
-    dog_loop_writer.Write(drivetrain_files[0], drivetrain_files[1])
+    dog_loop_writer.Write(drivetrain_files[0], drivetrain_files[1],
+                          drivetrain_files[2])
 
     kf_loop_writer = control_loop.ControlLoopWriter("KFDrivetrain", [
         kf_drivetrain_low_low, kf_drivetrain_low_high, kf_drivetrain_high_low,
@@ -533,7 +534,8 @@
     ],
                                                     namespaces=namespaces,
                                                     scalar_type=scalar_type)
-    kf_loop_writer.Write(kf_drivetrain_files[0], kf_drivetrain_files[1])
+    kf_loop_writer.Write(kf_drivetrain_files[0], kf_drivetrain_files[1],
+                         kf_drivetrain_files[2])
 
 
 def PlotDrivetrainSprint(drivetrain_params):