Back to using "365_day" as the default calendar.

Now PISM interprets it literally: run lengths, yearly and monthly
reporting uses 365 * 24 * 60 * 60 second-long years if the "365_day"
calendar is selected. Similarly for "360_day".

Max. time-step length is still converted from "years" to "seconds"
using UDUNITS-2.

Also: from now on periodic climate data supports period that are an
integer number of years *only*.