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.