To make the interface more clear this commit introduces a new
intersection interface and deprecates the old one.

The old inside(n) is replaced by targetEntity(n)
The old outide(n) is replaced by domainEntity(n)
The old neighbor() is replaces by
numTargetNeighbors()/numDomainNeighbors().

Furthermore num{}Neighbors returns 1 if the {}-dimension is
smaller than the other dimension.