Features and fixes

* added the trace function for a SquareMatrix

* added Vector3.hat() and it's counterpart Dcm.vee()
for skewsymskew symmetric matrix operations in relation to the cross product
see https://en.wikipedia.org/wiki/Hat_operator

* Matrix::write_string produced runtime errors when I used it in PX4 posix simulation, i simplified it

* a Matrix3f is a SquareMatrix

* added tests for SquareMatrix.trace, Vector3.hat and Dcm.vee

* added a test for quaternion initialisation from array

* preventing buffer overflows in Matrix.write_string method

matrix/Vector3.hpp

@@ -16,6 +16,9 @@ namespace matrix
 template <typename Type, size_t M>
 class Vector;
 
+template<typename Type>
+class Dcm;
+
 template<typename Type>
 class Vector3 : public Vector<Type, 3>
 {
@@ -61,6 +64,21 @@ public:
         return (*this).cross(b);
     }
 
+    Dcm<Type> hat() const {    // inverse to Dcm.vee() operation
+        const Vector3 &v(*this);
+        Dcm<Type> A;
+        A(0,0) = 0;
+        A(0,1) = -v(2);
+        A(0,2) = v(1);
+        A(1,0) = v(2);
+        A(1,1) = 0;
+        A(1,2) = -v(0);
+        A(2,0) = -v(1);
+        A(2,1) = v(0);
+        A(2,2) = 0;
+        return A;
+    }
+
 };
 
 typedef Vector3<float> Vector3f;