Simple helper method to convert from cv::Mat (32F) to Eigen::MatrixXf

Pull request per new guidelines. Can be reviewed by anyone but I will nominate @jklontz 

This is a shallow copy right? Isn't it only guaranteed while `m` is valid? In which case `m` should be passed by const reference and a note should be made that `m` needs to remain valid while the return value of this function is in use?