Add relative encoder zeroing strategy

Change-Id: I07b5461a9766aa25c9c858b7771af26ed08ad4ff
diff --git a/frc971/control_loops/control_loops.fbs b/frc971/control_loops/control_loops.fbs
index 4f1e888..a95d407 100644
--- a/frc971/control_loops/control_loops.fbs
+++ b/frc971/control_loops/control_loops.fbs
@@ -61,6 +61,14 @@
   absolute_encoder:double;
 }
 
+// Represents all of the data for a single encoder.
+// The relative encoder values are relative to where the encoder was at some
+// arbitrary point in time.
+table RelativePosition {
+  // Current position read from the encoder.
+  encoder:double;
+}
+
 // The internal state of a zeroing estimator.
 table EstimatorState {
   // If true, there has been a fatal error for the estimator.
@@ -105,6 +113,14 @@
   absolute_position:double;
 }
 
+table RelativeEncoderEstimatorState {
+  // If true, there has been a fatal error for the estimator.
+  error:bool;
+
+  // The estimated position of the joint.
+  position:double;
+}
+
 // The internal state of a zeroing estimator.
 table IndexEstimatorState {
   // If true, there has been a fatal error for the estimator.