lisa.datautils: Add df_delta() function

Created by: pierregondois

The df_delta function merges two dataframes of events that are intended to be consecutive, e.g. the first 'pre' event is emitted as the entry of a function, and a'post' event is emitted at the exit.

The function keeps all the columns of the 'pre' and 'post' dataframe. It adds an additional 'delta' column containing the duration between the emission of a 'pre' event and its consecutive 'post' event.

Signed-off-by: Pierre Gondois Pierre.Gondois@arm.com