ligolw_tisi:  remove RowsFromOffsetDict() function

- has been replaced by .append_offsetvector() method of TimeSlide table class
ligolw_tisi:  edit a comment
remove unused import of ligolw_tisi module
glue.ligolw.utils.ligolw_sqlite:  internal namespace adjustment
ligolw_sqlite:  detect missing input, don't create empty database