Extended verison of XFeat cpp with OpenCV

[ayushkumar8340]

This repo is the extended version of Xfeat. The main changes are mentioned below:

1. Added opencv data- types for keypoints and descriptors and to accommodate those certain changes have been made in the xfeat.cc file.
2. Extended the examples by adding realsense_demo.cc which uses a realsense camera for realtime feature detection and matching.
3. Added docker file for easy setup of the environment.
4. Fixed the tensortoMat function to accommodate for both keyframes and descriptors.
5. Added compatibility with Cmake 3.5 , Ubuntu 20.04 and OpenCV 4.1.1