Allow to ask for the number of entities with a negative codimension
Currently, an exception is thrown when OneDGrid is asked for the number of entities on a given level with a given negative codimension. AFAIK this behavior is not actually required by any formal specification. As the LagrangeBasis class from dune-functions currently does such calls (only to disregard the result later) I am changing OneDGrid such that the size(level,codim) method returns 0 for negative codimensions.