Restructuring model and reductor modules

[pmli]

src/pymor/models/iosys.py is quite long (currently, it has 3165 lines). It might be good to break it into multiple modules, e.g., lti, so, ph, delay, bilinear under pymor.models (or under pymor.models.iosys).

Related to that, it might be good to separate reductors by the type of model. There is already a pymor.reductors.ph module, so there could be similar ones for lti and so.