Add cmake opencv 4 support

[ambersun1234]

Add opencv4 support in cmake compilation process

opencv3 to 4 had changed some of the variable name
causing the compilation failed.
I've add some of the switch to overcome these kind of error

in CMakeLists.txt file, there is a option to enable build opencv4 or not

caffe_option(OPENCV_VERSION4 "Build with OpenCV4 support" ON)                 

[gineshidalgo99]

OpenPose does not need Caffe with OpenCV, so this PR is not high priority for me to be involved on it. However, I would be happy to accept it if the following changes are made:

• It should not be a CMake flag. This would require the user knowledge and manual switching (error prone). You can read the OpenCV version with C++ macros, and apply it automatically in the code without the CMake flag. I actually do this in OpenPose.
• I see "// #include "opencv2/imgcodecs/imgcodecs.hpp"" commented out all over the code. It should simply be removed then.
• Rather than using #define CV_LOAD_IMAGE_COLOR cv::IMREAD_COLOR in every single file, there should be a new header file that does the following:

  #ifdef OPENCV_VERSION4
    #define CV_LOAD_IMAGE_COLOR cv::IMREAD_COLOR
    #define CV_LOAD_IMAGE_GRAYSCALE cv::IMREAD_GRAYSCALE
  #endif

And then all other files should simply include this new header. (I also do this in OpenPose itself)

With the 3 changes above i'll happily merge it!

[ambersun1234]

@gineshidalgo99 All of the above issue had been corrected.