Change HallEffectAndPosition to proper naming

All of the templates assume the position variable is called encoder,
not position. I spent a while trying to figure out if the naming
should be the other way around, but I think 'encoder' is correct.

Change-Id: Icb0fa9a94bdea4ae3a43b947a073cc1f83ba5f4a
diff --git a/y2017/wpilib_interface.cc b/y2017/wpilib_interface.cc
index 01184fa..12f3313 100644
--- a/y2017/wpilib_interface.cc
+++ b/y2017/wpilib_interface.cc
@@ -453,7 +453,7 @@
                     double encoder_counts_per_revolution, double encoder_ratio,
                     bool reverse) {
     const double multiplier = reverse ? -1.0 : 1.0;
-    position->position =
+    position->encoder =
         multiplier * encoder_translate(counter.polled_encoder(),
                                        encoder_counts_per_revolution,
                                        encoder_ratio);