The C/C++ standard math library provides erfc().

(erfc() is a part of the C99 standard. It seems to be a lot faster
than GSL's gsl_sf_erfc(), too.)
clobber 2 of 3 pedantic warnings; see remaining in PA_paleo_precip.cc
Now periodicity parameters are arguments of IceModelVec2T::init().

Also: fixes pedantic compiler warnings ("unsigned int" vs "int").
Fixes a typo at iceModelVec2T.cc:246
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

(cherry-picked from 781868f (dev branch))