Add a clean time series method "average"

[ryandrewjones]

When I am doing reconciliation between top down and bottom up data and get a series of "factors" over time, I may want to to take an average of these factors to serve as reconciliation factors in future years.

To do this, I need a clean timeseries method called "average" that fills missing years with an average of the given years.

AC

1. the TimeSeries.clean method accepts an interpolation_method or extrapolation_method called "average" and uses the mean of the given data to fill in missing data over a new index

[ryandrewjones]

Ben, assigned to you to review and merge with master.