cumsum
fun cumsum(arr:Matrix<Double>):Matrix<Double>
Calculates the cumulative (ongoing) sum of a matrix's elements. For example,
cumsum(mat[1,2,3]) would return mat[1,3,6].
@param arr The matrix to calculate the cumsum on. Sum will be computed in row-major order.
@return A 1x(arr.numRows*arr.numCols) vector storing the ongoing cumsum.