[YaspGrid][Bugfix] Another fix for intersection movement code

This really isn't my day - the second movement actually happened in the
wrong direction.

I hope this finally settles the problem...
[Interface] Return Entity when reconstructing from EntitySeed

My earlier batch of EntityPointer -> Entity conversions missed one
interface method: Grid::entityPointer(const EntitySeed&), which
reconstructs an entity from its seed.

For this method, it does not make sense to simply change the return
type (its name would be rather misleading), so this patch adds a new
method Grid::entity(const EntitySeed&) and deprecates the old method.

I didn't add the call to CHECK_INTERFACE_IMPLEMENTATION because

a) I didn't want to break existing grids just now
b) I don't really understand that magic... ^^
[YaspGrid] Implement YaspGrid::entity()

This patch adds the new interface method for creating an Entity object
from an EntitySeed and deprecates the old method that returns an
EntityPointer.