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

test/attitude.cpp

@@ -231,6 +231,12 @@ int main()
     TEST(fabsf(q(2) - q_true(2)) < eps);
     TEST(fabsf(q(3) - q_true(3)) < eps);
 
+    // Quaternion initialisation per array
+    float q_array[] = {0.9833f, -0.0343f, -0.1060f, -0.1436f};
+    Quaternion<float>q_from_array(q_array);
+    for(int i = 0; i < 4; i++)
+        TEST(fabsf(q_from_array(i) - q_array[i]) < eps);
+
 };
 
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