nested_split splits by comma, keeps parenthesis sections together
See also
Other Data Cleaning Functions:
Check_Dupe_Columns(),
Check_Trunc(),
Check_Verbose(),
Date_Shift(),
Def_Control(),
Def_model_control(),
Event_Count_Gen(),
Event_Time_Gen(),
Joint_Multiple_Events(),
Replace_Missing(),
Time_Since(),
factorize(),
gen_time_dep(),
interact_them(),
parse_literal_string()