[FEA] Series level cumulative sum

[beckernick]

Is your feature request related to a problem? Please describe.
As a user, I want to calculate the cumulative sum of a series. I'd like it to behave like the pandas cumsum method, which can be found here.

Describe the solution you'd like
I'd like to be able to call series.cumsum() and return results consistent with what I would get from calling this on a pandas series.

Describe alternatives you've considered
The alternative is to iterate through each cell in the column, keep a running tally of the sum of values seen so far, and then store that value in a new array.

Additional context
I would think that column-level cumulative sums are more common than row-level cumulative sums, which would make supporting the row-level pattern (axis=1) less important.

[jrhemstad]

Necessary C++ functionality added in #1005