lisa.analysis.frequency: Fix df_peripheral_clock_effective_rate
lisa.analysis.frequency: Fix df_peripheral_clock_effective_rate

FIX

* Remove inplace modifications of dataframes returned by df_events() as
  the returned dataframes is cached and shared with other users.

* Simplify the "start" column creation using
  lisa.datautils.df_add_delta()

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