Interpret climate fields as piecewise-constant in time.

Also:
- fixes some bugs related to periodic forcing and re-setting snow
  depth at the beginning of the balance year

- allows choosing whether melted ice should refreeze using the
  "pdd_refreeze_ice_melt" configuration flag (default is "yes")

- improves PISMTime and PISMGregorianTime: adds calendar_year_start()
  and increment_date() methods