code: public static void extract( DMatrix src,
                                  int srcY0, int srcX0,
                                  DMatrix dst ) {
          extract(src,srcY0,srcY0+dst.getNumRows(),srcX0,srcX0+dst.getNumCols(),dst,0,0);
      }
docs: <p>
  Extracts a submatrix from 'src' and inserts it in a submatrix in 'dst'. Uses the shape of dst
  to determine the size of the matrix extracted.
  </p>
  
  @param src The original matrix which is to be copied.  Not modified.
  @param srcY0 Start row in src.
  @param srcX0 Start column in src.
  @param dst Where the matrix is extracted into.