Improved comparison operators for TriaAccessorBase

The previous commit was a bit too aggressive because it is still useful to catch the case when comparing iterators to two different triangulations (leading to infinite loops).

Therefore, I now merely extended the assertion to not trigger when one of the two objects is default constructed, i.e., does contain a null pointer to tria. The comparison then does not need tria == a.tria because invalid iterators use invalid cell and level index, too (leading to !=).