SubnetworkLayer: deprecate `from` with `concat_sources=False`

[albertz]

I think this depends on #530.

In case of concat_sources=False, the source names data:0 etc are not nice, and I think we can have better ways (this is #530).

(I'm not sure if we should also deprecate from for concat_sources=True, or if this is fine.)

[albertz]

Actually, not sure if the old behavior should be completely disallowed (so it needs a new behavior_version #508) or just deprecated.