FileStorage read/write for cv::Size,Rect,Vec (Bug #3196)

FileStorage can write cv::Size,Rect,Vec fine but is unable to read them back in directly.

Either the attributes must be written separately then read back in and used with a constructor or they can be read as a std::vector.

eg.

// WRITE
cv::Size s(2,2);
cv::Vec3f v(1.,1.,1.);

cv::FileStorage fs;
fs.open("test.xml", cv::FileStorage::WRITE);

fs << "s" << s;
fs << "v" << v;

fs.release();

// READ
cv::FileStorage fs2;
fs2.open("test.xml", cv::FileStorage::READ);

cv::Size s;
cv::Vec3f v;

fs["s"] >> s; // this doesn't work
fs["v"] >> v; // neither does this

fs2.release();

/** test.xml **/

<opencv_storage>
<s>
2 2</s>
<v>
1. 1. 1.</v>
</opencv_storage>