Switch from gsl_sf_erfc() to the C library erfc().

(It is faster this way.)
"-surface pdd_old" should use new variable names.

"climatic_mass_balance" instead of "acab" and "ice_surface_temp" instead
of "artm".
Fix the -..._times code.

With the old code PISM would run out of RAM when given "-ts_file ts.nc
-ts_times -1e12:1:1e12". This change protects us from this.

(One should use "-ts_file ts.nc -ts_times 1" instead, which means "every
year for the duration of the run.")