(re)define behaviour of Corpora.{file_ids,annotaion_names} for len(corpora) == 0
import Corpora from annolytics
cs = Corpora()
cs.file_ids([ID1234])
> corpora.py,file_ids(),ln 67: ignoring file_id(s):['ID1234']: not in all corpora.
> corpora.py,file_ids(),ln 71: continuing with file_id(s):[] for corpora.