complementary_filter: move const initializations out of header

Initialization of static consts other than int (here: float) inside the
class declaration is not permitted in C++. It works in gcc (due to a
non-standard extension), but throws an error in C++11.

imu_complementary_filter/include/imu_complementary_filter/complementary_filter.h

@@ -89,12 +89,12 @@ class ComplementaryFilter
                 double dt);
 
   private:
-    static const double kGravity = 9.81;
-    static const double gamma_ = 0.01;
+    static const double kGravity;
+    static const double gamma_;
     // Bias estimation steady state thresholds
-    static const double kAngularVelocityThreshold = 0.2;
-    static const double kAccelerationThreshold = 0.1;
-    static const double kDeltaAngularVelocityThreshold = 0.01;
+    static const double kAngularVelocityThreshold;
+    static const double kAccelerationThreshold;
+    static const double kDeltaAngularVelocityThreshold;
 
     // Gain parameter for the complementary filter, belongs in [0, 1].
     double gain_acc_;

imu_complementary_filter/src/complementary_filter.cpp

@@ -37,6 +37,13 @@
 
 namespace imu_tools {
 
+const double ComplementaryFilter::kGravity = 9.81;
+const double ComplementaryFilter::gamma_ = 0.01;
+// Bias estimation steady state thresholds
+const double ComplementaryFilter::kAngularVelocityThreshold = 0.2;
+const double ComplementaryFilter::kAccelerationThreshold = 0.1;
+const double ComplementaryFilter::kDeltaAngularVelocityThreshold = 0.01;
+
 ComplementaryFilter::ComplementaryFilter() :
     gain_acc_(0.01),
     gain_mag_(0.01),