Restructure yaspgridentity.hh

Until now, the default implementation threw exceptions on
all function calls. Now, the exisiting codim == dim
specialization got the new default implementation. This
implementation is to be modified subsequently for arbitrary
codimension. The codim==0 implementation is not touched at
the moment.
Expand the entity seed for arbitrary codimension

The seed needs some information to distinguish between entities
on the same cell, but with different shift vector. The easiest
(and smallest datawise) solution seems to be to give an int, that
specifies which YGC::Iterator to use in YGrid. The coorect indexing
is dependent on the insertion order of the YGridComponents. makelevel
has to make sure to pick shift vecotrs in a consistent manner!