Consolidate naming of NIJZ files

[jurijbajzelj]

NIJZ does not name their XLS file consistently: the most obvious change is that they use _ sometimes and not other times. The nijz_{daily|weekly}.py script then processes wrong file due to order of files in COVID-DATA/EPI folder.

Currenly we workaround this manually by renaming files to fix the sort order and re-run the script.

Thread where discussion started:
https://sledilnik.slack.com/archives/C01DV090C0G/p1611147960016200