Implement stereo calibration using AprilTags

[sourishg]

Instead of using checkerboards, AprilTags can also be used to calibrate a pair of stereo cameras (may often lead to a more accurate calibration result).

Find correspondences using AprilTags instead of a checkerboard. An AprilTags calibration board looks something like this

C++ code for AprilTags can be found here: http://people.csail.mit.edu/kaess/apriltags/

[piyush-jena]

I would like to work on the issue.

[mpdmanash]

@theSarge911, great. You can start by implementing stand-alone apriltags package from http://people.csail.mit.edu/kaess/apriltags/ .

Then, you can add the apriltag corner detection from the library to replace the checkerboard corner detection in the calib_intrinsic.cpp and calib_stereo.cpp files. You would need to add a popt option to decide whether to select between the two types of corner detection.