Std unit conversions (#259)

* Added conversion constants

* implemented std unit conversion

* changed centerToFront ratio

Changed weight distribution ratios used to calc center of gravity distances to align closer to manufacturer specs

* implemented std unit conversion

* remove unused conversion

* reverted wheelbase conversion

slight change to pilot wheelbase

* removed redundant conversion

* removed incorrect/unused conversion

* removed class that now exists in honda/values.py

* redirect Cruisebuttons call

* redirect Cruisebuttons call

* Update interface.py

* Update numpy_fast.py

Refactor

* Update numpy_fast.py

* Update numpy_fast.py

-encapsulated get_interp 
-reduced calls to len() for iterable input

selfdrive/test/plant/plant_ui.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python
 import pygame
 from plant import Plant
-from selfdrive.config import CruiseButtons
+from selfdrive.car.honda.values import CruiseButtons
 import numpy as np
 import selfdrive.messaging as messaging
 import math

selfdrive/test/tests/plant/test_longitudinal.py

@@ -9,7 +9,8 @@ import shutil
 import matplotlib
 matplotlib.use('svg')
 
-from selfdrive.config import Conversions as CV, CruiseButtons as CB
+from selfdrive.config import Conversions as CV
+from selfdrive.car.honda.values import CruiseButtons as CB
 from selfdrive.test.plant.maneuver import Maneuver
 import selfdrive.manager as manager
 from common.params import Params