title: Mat* input for Cvknearest
body: <p>as we know CvKnearest neighbor get input in this manner</p>
  
  <p><code>CvKNearest::CvKNearest(const Mat&amp; trainData, const Mat&amp; responses, const Mat&amp;     sampleIdx=Mat(), bool isRegression=false, int max_k=32 )
  </code><br>
  or in this manner</p>
  
  <p><code>CvKNearest::CvKNearest(const CvMat* trainData, const CvMat* responses, const CvMat* sampleIdx=0, bool isRegression=false, int max_k=32 )
  </code></p>
  
  <p>But I have two cv::Mat* for trainData and response. how can i convert them to match one of these two input format for CvKnearest?  </p>