invoke = function invoke(v, i, j) {
      // invoke callback
      return callback(v, [i, j], me);
    }
Create a new matrix with the results of the callback function executed on
each entry of the matrix.
@memberof SparseMatrix
@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.
@param {boolean} [skipZeros] Invoke callback function for non-zero values only.

@return {SparseMatrix} matrix