Allow comparison == and != of TriaAccessorBase with different tria

It is well-defined to check for equality and inequality of TriaAccessorBase (cell_iterator types) that belong to different triangulations. Those are simply unequal. This is necessary e.g. for comparing with a default constructed cell_iterator variable. Of course, operator < is still undefined.
Merge pull request #262 from kronbichler/master

Allow comparison == and != of TriaAccessorBase with different tria