add capability hasEntityIterator

As decided on the 2014 developer meeting, this patch adds a new capability hasEntityIterator and implements it for all current grid implementations (using the same values as the hasEntity capability).

This closes #38 (closed).

changed milestone to %DUNE 2.6.0

added feature label

@oliver.sander @ansgar What should this say for UG? AFAIK, UG has level iterators for all codimensions, but not leaf iterators...

That's not correct. UG has both level and leaf iterators for elements and vertices, and no others.

That's good to know. I just had a vague memory from when I looked at the iterators a long time ago.

mentioned in commit e50f64ae

merged

mentioned in commit f559cd15

mentioned in merge request !239 (merged)