Really fix GridFunctionSpace fixed size detection when alternating betwenn 0 and non-zero sizes

In order to really fix this bug (see #59 (closed), #116 (closed), !129 (merged)), we have to introduce a separate state for geometry types that have not been visited at all. This patch adds a constant GT_UNUSED for that purpose. This resolves the ambiguity in the current code which used a size of 0 to signal that a geometry type has either not been visited yet or that it actually had size 0 for some entity.

Fixes #116 (closed)

Backport to releases/2.6