Add diff/pctChange to Series

[joshuakoh1]

diff and pctChange features added in https://github.com/javascriptdata/danfojs/pull/418 https://github.com/javascriptdata/danfojs/pull/414 only extend to DataFrames. Currently unable to apply to a single column slice without adding extra steps.

[risenW]

@NeonSpork Would you like to work on this?

[NeonSpork]

I think we need more information specifically about what you're trying to achieve that's failing, and how @joshuakoh1 ?

The diff/pctChange functions should mirror the behavior of the other math functions like pow etc, and it should accept a single number, series, column, etc, already.

Can you provide example code, errors and more detailed information about what behavior you expected please?

[joshuakoh1]

@NeonSpork Just trying to calc the diff/pctChange for a single series instead of the full DataFrame. Would like to be able to do df.column.pctChange(1) instead of df.pctChange(1).column. The latter performs unnecessary operations.