Bug 8800 - useDaysMode=Datedue wrong behaviour (revisited)

Bug 8800 - useDaysMode=Datedue wrong behaviour (revisited)

useDaysMode=Datedue wasn't used as advertised in the docs. Added
next_open_day and prev_open_day subs to Koha::Calendar and some tests for them.

- Koha::Calendar->addDate was rewritten in a more sane way (also split into
  addHours and addDays for convenience).
- Fixed a bug introduced in Bug 8966 regarding dt truncation and dtSets->contains
- Minor docs typos
- Use the passed Calendar mode or default to 'Calendar' in Koha::Calendar->_mockinit.
- Tests

I'm writing some db-dependent tests for is_holiday, and hopefully for CalcDateDue so any
rewrite/followup doesn't break things.

Regards
To+

Sponsored-by: Universidad Nacional de Córdoba
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passed-QA-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>