enum should be able to take list for some parameters

[FabienGiely]

parameters "sep", "end_of_bullet" should take string or list

nlg_enum(
   ["elem1", "elem2", "elem3", "elem4", "elem5"],
   sep=["sep1", "sep2"]
)

Elem1 sep1 elem2 sep2 elem3 sep1 elem4 and elem5