add: GravityFilter

pasada-lib/include/step_detector.h

@@ -48,4 +48,17 @@ public:
     void primeFilters(double fps, std::vector<double> sig);
 };
 
+/** mean-filter the gravity vector, then take acceleration downwards */
+class GravityFilter {
+    size_t N;
+    std::vector<double> gauss_taps;
+    Filt gx;
+    Filt gy;
+    Filt gz;
+public:
+    // 5 secs buffer, prime y with direction of gravity (for tests & faster init)
+    GravityFilter(double fps);
+    double filter(std::vector<float> values);
+};
+
 #endif //PASADASUPERPROJECT_STEP_DETECTOR_H