recurse = function recurse(value, index) {
      if (isArray(value)) {
        value.forEach(function (child, i) {
          recurse(child, index.concat(i));
        });
      } else {
        callback(value, index, me);
      }
    }
Execute a callback function on each entry of the matrix.
@memberof DenseMatrix
@param {Function} callback   The callback function is invoked with three
                             parameters: the value of the element, the index
                             of the element, and the Matrix being traversed.