lisa.datautils: Revert on pd.concat in df_merge() when necessary
lisa.datautils: Revert on pd.concat in df_merge() when necessary

If columns have the same name, revert to using pd.concat() followed by
an index sort rather than using the potentially faster DataFrame.join().

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