Crossing IDs

In different rooms, subrooms can have the same id, crossings can't.

Now:

room 1, subrooms 1-4, crossings 1-3
room 2, subrooms 1-4, crossings 4-6

Better:

room 1, subrooms 1-4, crossings 1-3
room 2, subrooms 1-4, crossings 1-3

So you are more flexible in writing geometry files.