Whitespace changes.
Fix an unreported bug. Port to 0.6.

3D diagnostic quantities computed on demand did not work with automatic
vertical grid extension: saving "cts" could make PISM create an output
file with the number of z levels corresponding to the old vertical
grid (before automatic extension). This lead to a crash when a properly
"vertically extended" field was saved.
Use x0,y0 and Lx,Ly in grid_info (instead of {x,y}_{min_max}).
Use the xy-periodic grid by default.
Use non-periodic grid in the symmetry test.
Update the vertical interpolation (regridding) test.
Better grid coordinate computation.

The grid is centered with respect to (x0,y0) now.
Use the old default (non-periodic grid) in regression tests.

I could update all the verification test error reports stored in
regression scripts, but this is easier.
Fix #277. Port to 0.6.
Add OPTIONAL_FILL_MISSING and CRITICAL_FILL_MISSING regridding flags.

Use these to replace missing values matching the _FillValue attribute
with a provided default value.

See IceModelVec::regrid() for details.

There is no way to do this from the command line, and this does not
affect user-observable behavior.

PS: This is to let hydrology models read in the sliding speed, which in
    PISM output files has missing values in ice-free areas.

PPS: In a regular run PISM should probably stop with an error message if
     a variable contains missing values. What do you think?
Fix a stupid bug in the fix for an unreported bug.

(See b102325.)
Merge branch 'dev' into hydro-simplify-transfer
Use CRITICAL_FILL_MISSING flag and fix committed by ckhroulev in 1f14abf.  See 23dda01.  Works for me.