ice_calendar: fix hourly outputs with 'histfreq_n /= 1'

In b720380 (Update Time Manager (CICE-Consortium#566), 2021-03-16), the computation of
'elapsed_hours' in ice_calendar::calendar was changed from

      elapsed_hours = int(ttime/3600)

to

      elapsed_hours = elapsed_days * hours_per_day

In the previous version, 'ttime' held the total number of seconds since
the beginning of the run, such that 'elapsed_hours' correctly held the
number of elapsed hours since the beginning of the run. Howeve, the new
computation is incorrect; it does not take into account the hours
elapsed into the current date, nor the fact that the run may have
started at a different time than 00:00.

This in turn leads to hourly outputs being written each hour even if
'histfreq_n /= 1'.

Fix the computation.

cicecore/shared/ice_calendar.F90

@@ -354,7 +354,7 @@ subroutine calendar()
       elapsed_years = myear - year_init
       elapsed_months = elapsed_years*months_per_year + mmonth - month_init
       elapsed_days = compute_days_between(year_init,month_init,day_init,myear,mmonth,mday)
-      elapsed_hours = elapsed_days * hours_per_day
+      elapsed_hours = elapsed_days * hours_per_day + hour - sec_init / seconds_per_hour
       call calendar_date2time(myear,mmonth,mday,msec,timesecs)
 
       !--- compute other stuff