SummationByPart's Diff_gv aliased function does ont document which part of the grid the computed derivative is valid

Issue #2633 new
Roland Haas created an issue

Looking at the code in SummationByParts

```  case (0) direction
if ( bb(1) == 0 ) then
il = 1 + gsize                                                                                        else
ol = offset(1)
!\$omp parallel workshare
dvar(1+ol,:,:) = ( q(1,1) * var(1+ol,:,:) + q(2,1) * var(2+ol,:,:) ) * idel
dvar(2+ol,:,:) = ( q(1,2) * var(1+ol,:,:) + q(3,2) * var(3+ol,:,:) ) * idel
!\$omp end parallel workshare
il = 3 + ol
end if
```

seems to indicate that for non-boundary ends of a grid component (`bb == 0`) derivatives are computed only in the interior (from `il = 1+gsize`) while for boundary ends some (larger) area is valid.

This does not seem to be documented though, at least I could not find it in either documentation.tex nor interface.ccl.