First attempt at ripping out agnostic code into ifcopenshell.api namespace. See #1399.

src/ifcopenshell-python/ifcopenshell/api/geometry/assign_styles.py

@@ -0,0 +1,31 @@
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "shape_representation": None,
+            "styles": [],
+            "should_use_presentation_style_assignment": False,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        if not self.settings["styles"]:
+            return []
+        self.results = []
+        for element in self.file.traverse(self.settings["shape_representation"]):
+            if not element.is_a("IfcShapeRepresentation"):
+                continue
+            for item in element.Items:
+                if not item.is_a("IfcGeometricRepresentationItem"):
+                    continue
+                style = self.settings["styles"].pop(0)
+                name = style.Name
+                if self.file.schema == "IFC2X3" or self.settings["should_use_presentation_style_assignment"]:
+                    style = self.file.createIfcPresentationStyleAssignment([style])
+                self.results.append(
+                    self.file.createIfcStyledItem(
+                        item, [style], name
+                    )
+                )
+        return self.results