readGrid and writeGrid removed from grid interface and moved to

GridDefault class. Default implementation is doing nothing.

Furthermore removed BoundaryType and moved to DiscreteFunctionSpace in
dune/fem/common.

BoundaryEntity implementation moved to grid.cc

Added subBoundaryId method on EntityDefault which work just the same way as
subIndex but for the method boundaryId.

Added GridName which returns name of grid for given GridIdentifier.

[[Imported from SVN: r1163]]