* 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
