added interface for walkers

some new helper methods
added interpolater for paths

grid/walk/GridWalkHelper.h

@@ -2,6 +2,7 @@
 #define GRIDWALKHELPER_H
 
 #include "../../geo/Heading.h"
+#include "GridWalkState.h"
 
 class GridWalkHelper {
 
@@ -41,6 +42,7 @@ public:
 	template <typename T, typename Walker> static GridWalkState<T> retryOrInvert(Walker& w, const int numRetries, Grid<T>& grid, GridWalkState<T> start, float distance_m) {
 
 		Assert::isTrue(distance_m >= 0, "distance must not be negative!");
+		Assert::isNotNull(start.node, "starting node must not be null");
 
 		GridWalkState<T> res;