CAD11 — CadMain::Model interface issue

CADServices 1.1
1. In The CadMain::Model interface there is no clear way to get the DesignFeatures of a Model. The only possible way would be via top_level_entities() or unique_entities(), but these calls are for geometric entities (so it is written in the documentation).
Proposed Solution: add a call "CadFeature::DesignFeatureSeq design_features()" to CadMain::Model. The DesignFeatures should be more clearly seperated from the BrepEntities as they are to different sights on the same data and should not be mixed per chance.