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/Dcm.hpp

@@ -83,6 +83,15 @@ public:
         dcm(2, 1) = sinPhi * cosThe;
         dcm(2, 2) = cosPhi * cosThe;
     }
+
+    Vector<Type, 3> vee() const {    // inverse to Vector.hat() operation
+        const Dcm &A(*this);
+        Vector<Type, 3> v;
+        v(0) = -A(1,2);
+        v(1) =  A(0,2);
+        v(2) = -A(0,1);
+        return v;
+    }
 };
 
 typedef Dcm<float> Dcmf;