Argument check in EdgeIterator inner constructor

[vlc1]

The current inner constructor for EdgeIterator throws an error when the inner field is empty:

julia> inner = CartesianIndices((0,));

julia> outer = CartesianIndices((1,));

julia> EdgeIterator(outer, inner)
ERROR: DimensionMismatch: CartesianIndices((0,)) must be in the interior of CartesianIndices((1,))

Fix calls issubset instead.