Add NOK to riversystem ELBE

To address your commit: 97d0d97d2cd61caa96ae2ab78fd9ef55a0103ea9.

I went further, adding the whole Nord Ostsee Kanal to the river system Elbe by some more specific divides.
I think this would describe the situation better and associate a lot of small rivers and drains to the basin too.

These are my first steps with git and github, so feel free to drop some critics!

By the way: i found a quite simple workaround to add coastlines to basins. Are you interested?