Add stepwise controller

This adds the ability to use stepwise curves alongside
pid control. This creates a base controller class that
pidcontroller and stepwise controller inherit from.

Note: Hysteresis to come in follow-on patch

Tested-by:
Created a stepwise controller and noticed that when it
crossed a threshold that it contributed to the pwm setting.

Change-Id: I6cf842f80eaccafc905d620970afe91e2092d568
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/pid/pidthread.cpp b/pid/pidthread.cpp
index 490b70e..4dfc22a 100644
--- a/pid/pidthread.cpp
+++ b/pid/pidthread.cpp
@@ -16,7 +16,7 @@
 
 #include "pidthread.hpp"
 
-#include "pid/controller.hpp"
+#include "pid/pidcontroller.hpp"
 #include "sensors/sensor.hpp"
 
 #include <chrono>