Bug/astral valuerror

Solves the various issues with days that have 0 or > 1 dusks. Admittedly it has some shortcuts, but since we are not interested in minute precision I think we can live with them.

I would like also to reopen the discussion about how we deal with timestamps corresponding to daytime. Currently we raise a ValueError. I think this is not the correct behaviour, I mean CalibPipe shouldn't decide whether to stop processing if a timestamp is not during dark time, right?

Closes #112 (closed)