Fix date2str when passed a date.
Merge remote-tracking branch 'refs/remotes/yade.usd/develop' into develop
Add the normalize_datestr and datetime.

Both are actually aliases of date2str and dt2str, which allow now
strings to be provided and use `normalize_date` and `normalize_datetime`
internally.
Merge branch 'develop' of /media/manu/GRANDEROJA/xoeuf into develop

Conflicts:
	xoeuf/tools/__init__.py