Simple extrapolation API

[gdementen]

I.e. fill missing data points after non-missing data points.

See:
http://stackoverflow.com/questions/22491628/extrapolate-values-in-pandas-dataframe/35959909#35959909

Pandas supports interpolation natively (ie fill missing data points between non-missing data points).

http://pandas.pydata.org/pandas-docs/stable/missing_data.html#interpolation
http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.interpolate.html#pandas-dataframe-interpolate