UnstructuredGrid ignores offset in c-tor

[gentryx]

An UnstructuredGrid (and likely UnstructuredSoAGrid, too) will only honor the dimension in a CoordBox argument to its constructor. Consequence: instead of storing cells 2, 3, 4, 5, 6, 7 a grid will store cells 0, 1, 2, 3, 4, 5 -- one of the reasons for the segfaults we've seen with dgswem.

Proposed workaround: proper ID renumbering.