lisa.analysis.tasks: Remove duplicates in df_task_activation
lisa.analysis.tasks: Remove duplicates in df_task_activation

Make sure there is no duplicated runs of rows in the dataframe, to allow easy
computation of time spent in each activation.

credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa