clarify relation of output channels, exit nodes and segments

[ichthyo]

still some fuzziness in here; probably due to the fact that I discovered that it's impossible to keep the topic "multichannel" entirely out of the low-level model (the reason being compressors and spatial sound and image handling)

[ichthyo]

next things to to

[ichthyo]

(In #718) probably knowing enough now about that topic to try to nail it down.....

[ichthyo]

Actually this work is done.

The key for solving this was the introduction of the entity ModelPort.

• a channel as such isn't relevant anymore. This will be settled by the stream type system
• an elementary stream ends up being hooked up to a (global) Pipe
• each pipe not connected any further yields a ModelPort. This will be detected by the builder
• on top level, there is a partitioning by timeline; below of that is the temporal segementation
• within each segment, for each ModelPort there is a corresponding exit node.

While the definition work is done, I'll leave this around as meta ticket to track the progress of the actual datastructure definition