lisa: Introduce datautils module
Contains generic helpers for dataframe, series and more generally data manipulation.
- df_merge replaces concat+sort_index so that both are done in one go, which should be more efficient
- series_derivate allows working on dataframe columns (Series) easily.
The naming convention of functions is an extension of the one of analysis: prefix functions with the kind of things it operates on (df_
, series_
etc).