Add a 4 point spline to 6 point spline function

This makes a 6th order spline with the same shape as the provided 4th
order spline.

Change-Id: I14cf6ca0560c5a99ece2152c444dc7509dc7a34f
diff --git a/frc971/control_loops/drivetrain/spline.h b/frc971/control_loops/drivetrain/spline.h
index 7a77082..c5b94cc 100644
--- a/frc971/control_loops/drivetrain/spline.h
+++ b/frc971/control_loops/drivetrain/spline.h
@@ -150,6 +150,10 @@
            1.0 / magdxy2 * (dx * dddy - dy * dddx);
   }
 
+  const ::Eigen::Matrix<double, 2, N> &control_points() const {
+    return control_points_;
+  }
+
  private:
   const ::Eigen::Matrix<double, 2, N> control_points_;
 
@@ -163,6 +167,10 @@
 
 typedef NSpline<4> Spline;
 
+// Converts a 4 control point spline into
+::Eigen::Matrix<double, 2, 6> Spline4To6(
+    const ::Eigen::Matrix<double, 2, 4> &control_points);
+
 }  // namespace drivetrain
 }  // namespace control_loops
 }  // namespace frc971