Implemented merge of the objects to the one multi-part object

+ Implemented merge of the parts to the one object. But now this function doesn't used.

+ Fixed Model::looks_like_imperial_units()

src/libslic3r/Model.hpp

@@ -287,6 +287,7 @@ public:
     bool needed_repair() const;
     ModelObjectPtrs cut(size_t instance, coordf_t z, bool keep_upper = true, bool keep_lower = true, bool rotate_lower = false);    // Note: z is in world coordinates
     void split(ModelObjectPtrs* new_objects);
+    void merge();
     // Support for non-uniform scaling of instances. If an instance is rotated by angles, which are not multiples of ninety degrees,
     // then the scaling in world coordinate system is not representable by the Geometry::Transformation structure.
     // This situation is solved by baking in the instance transformation into the mesh vertices.