Tidy up time calculation in ice_history_write

[anton-seaice]

At the moment, we calculate the the middle of history interval time as:

! set timestamp in middle of time interval
        if (histfreq(ns) == 'm' .or. histfreq(ns) == 'M') then
            if (month /= 1) then
                ltime=time/int(secday)-real(daymo(month-1))/2.0
            else
                ltime=time/int(secday)-real(daymo(12))/2.0
            endif
        else if(histfreq(ns) == 'd' .or. histfreq(ns) == 'D') then 
            ltime=time/int(secday) - 0.5
        else
            ltime=time/int(secday)
        endif

We can probably replace this with:

if (hist_avg) then
    ltime = p50*(time_beg(ns) + time_end(ns))
else
    ltime = time_end(ns)
endif()

in the ice_history_write.F90 files