Switching to UDUNITS-2.
Replaces "m/a" and "m a-1" with "m/year".

(UDUNITS-2 interprets "a" as "are" (100 m2).)
Adds CalCalcs to the PISM source tree. Fixes FindUDUNITS2.cmake.
Switches to UDUNITS-2 completely. (See #148 and #60).
Tweaks in the build system.

This should help PISM find UDUNITS on fish.
Updates the Installation Manual.
FindUDUNITS2.cmake also looks for EXPAT (if needed).
Rename: UDUNITS2_LIBRARY -> UDUNITS2_LIBRARIES.
Implemented command-line options for both modes (with and without calendars).
Better code (isolated conditions validating user inputs).
Adds a forgotten call to PISMTime::init().
Finishing touches. Documented new time-management code.

(Comments are welcome.)
Fixes stupid bugs (plus some cleanup).

(Critical code should be compiled in *both* debugging and optimized modes.)
Merge branch 'udunits2' into dev