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/hatvee.cpp

@@ -0,0 +1,23 @@
+#include <stdio.h>
+#include "test_macros.hpp"
+
+#include <matrix/math.hpp>
+
+using namespace matrix;
+
+template class SquareMatrix<float, 3>;
+
+int main()
+{
+    Euler<float> euler(0.1f, 0.2f, 0.3f);
+    Dcm<float> R(euler);
+    Dcm<float> skew = R - R.T();
+    Vector3<float> w = skew.vee();
+    Vector3<float> w_check(0.1348f, 0.4170f, 0.5647f);
+
+    TEST(isEqual(w, w_check));
+    TEST(isEqual(skew, w.hat()));
+    return 0;
+}
+
+/* vim: set et fenc=utf-8 ff=unix sts=0 sw=4 ts=4 : */