Pad the year in time.reference_date with zeros (see #533)

CF conventions say that the form of the reference date is defined by UDUNITS.

UDUNITS says that the "date" in "time_unit since date" has the form be Y-M-D, where

- Y consists of 1,2,3 or 4 digits,
- M is (possibly zero-padded) month number
- D is (possibly zero-padded) day number

PISM uses UDUNITS and CalCalcs by David W. Pierce to handle time units, but CalCalcs
does *not* parse units strings itself (it uses the "ut_unit" data type from UDUNITS), so
zero-padding will not affect PISM's use of CalCalcs.

In summary: zero-padding years in the reference date should not affect anything except
xarray.

src/pism_config.cdl

@@ -2714,7 +2714,7 @@ netcdf pism_config {
     pism_config:time.holocene_start_type = "number";
     pism_config:time.holocene_start_units = "years";
 
-    pism_config:time.reference_date = "1-1-1";
+    pism_config:time.reference_date = "0001-1-1";
     pism_config:time.reference_date_doc = "year-month-day; reference date used for calendar computations and in PISM output files";
     pism_config:time.reference_date_type = "string";