Now periodicity parameters are arguments of IceModelVec2T::init().

Also: fixes pedantic compiler warnings ("unsigned int" vs "int").
Classes derived from PGivenClimate use IceModelVec2T::average() in update().
Fixes a typo at iceModelVec2T.cc:246
PDD model: melted ice should not refreeze (port to 0.5)

A one-line change to correct what I believe to be a small mistake in the new PDD code.
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
Cleaning up the doxygen browser build warnings.
Updates the documentation for "-surface pdd" (in dev).
This should fix a (recently introduced) bug in the lapse_rates modifier.
Merge branch 'dev' into hydro-add-melt
minor: add doc to ross example