Implement a shift function

[femtotrader]

Hello,

a shift function similar to https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.shift.html could be useful.

A pct_chg function could be useful also
http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.pct_change.html

Kind regard

PS: could help #1

[dysonance]

I believe the lag function is what you're probably looking for, though I can quickly add a shift method to just alias the lag function.

As for percentage changes, you can do diff(log(X)) for continuous, or diff(X, pad=true, padval=NaN) ./ X for discrete.

This does indicate another need with docs though, which is just the need to get more of the basic functionality documented somewhere.

[dysonance]

Pull request to address this opened here: #15

[dysonance]

PR above has been merged, closing now, let me know if more to be done.