Add hysteresis to stepwise controller

Tested-by: Ran on platform monitoring output and wrote
unit test

Change-Id: I74a1d21544c1a9cb4c1cb26dd4a353cbff0442d0
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/pid/ec/stepwise.hpp b/pid/ec/stepwise.hpp
index ed07b44..4034b47 100644
--- a/pid/ec/stepwise.hpp
+++ b/pid/ec/stepwise.hpp
@@ -29,6 +29,8 @@
     float ts; // sample time in seconds
     float reading[maxStepwisePoints];
     float output[maxStepwisePoints];
+    float positiveHysteresis;
+    float negativeHysteresis;
 };
 
 float stepwise(const ec::StepwiseInfo& info, float value);