Add middle helper to Range for absolute encoder.

Change-Id: I67219096dd1179e88bf444fbbf6a228135486b21
diff --git a/frc971/constants.h b/frc971/constants.h
index fc3e12b..a52add2 100644
--- a/frc971/constants.h
+++ b/frc971/constants.h
@@ -96,6 +96,8 @@
   double upper_hard;
   double lower;
   double upper;
+
+  double middle() const { return (lower_hard + upper_hard) / 2.0; }
 };
 
 }  // namespace constants