Added model placeholders

src/models/company_model.py

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+from src.models.abstract_reference import abstract_reference
+
+class company_model (abstract_reference):
+    pass

src/models/measurement_unit_model.py

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+from src.models.abstract_reference import abstract_reference
+
+class measurement_unit_model (abstract_reference):
+    pass

src/models/nomenclature_group_model.py

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+from src.models.abstract_reference import abstract_reference
+
+class nomenclature_group_model (abstract_reference):
+    pass

src/models/nomenclature_model.py

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+from src.models.abstract_reference import abstract_reference
+
+class nomenclature_model(abstract_reference):
+    pass

src/models/warehouse_model.py

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+from src.models.abstract_reference import abstract_reference
+
+class warehouse_model (abstract_reference):
+    pass