get(rowIndex, columnIndex) { // eslint-disable-line no-unused-vars
                throw new Error('get method is unimplemented');
            }
Returns the given element of the matrix. mat.get(3,4) is equivalent to matrix[3][4]
@abstract
@param {number} rowIndex - Index of the row
@param {number} columnIndex - Index of the column
@return {number}