Improve gyro zeroing

Instead of using a single large data chunk for zeroing, collect multiple
shorter samples that we then gradually accumulate.

Change-Id: I5496a7c47a703764fcd0f5b1498ef080e9b30fe9
diff --git a/frc971/zeroing/averager.h b/frc971/zeroing/averager.h
index b6ff09d..23a1b06 100644
--- a/frc971/zeroing/averager.h
+++ b/frc971/zeroing/averager.h
@@ -19,6 +19,11 @@
 class Averager {
  public:
   typedef Eigen::Matrix<Scalar, rows_per_sample, 1> Vector;
+  Averager() {
+    for (Vector &datum : data_) {
+      datum.setZero();
+    }
+  }
   // Adds one data point to the set of data points to be averaged.
   // If more than "num_samples" samples are added, they will start overwriting
   // the oldest ones.