external model interface

[ichthyo]

factor out a export interface of the high-level model, cleanly separated from the implementation.

Ideally, it should be usable without linking against liblumieraproc.so

[ichthyo]

(In #44) Meanwhile, I'm understanding the situation much better.

• I was able to remove all dependencies from lib. This essentially breaks the cycle between the libraries.
• but the common (application core) will always necessarily depend on both proc and backend. The reason are the data types used on the Session and Engine interfaces.
• if we want to avoid the latter, we'd need to do some magic dance with an transparent proxy layer and export interfaces using only placeholder datatypes.

I don't see any reason why to put any effort into the latter right now. Thus closing as wontfix