Fix RACMO surface mass balance missing value issue.  Port to 0.6.

In the std-greenland example it was quite a big error to keep the
RACMO smb=0 fill value, which affected most runs in section 1 of the User's
Manual.  This fix is a reasonable choice instead: -1 m/a ablation where
smb is missing.
Fixes to figures in User Manual resulting from change in previous commit; figures 3--9 fixed.  Text updated.