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

src/ifcopenshell-python/ifcopenshell/api/material/reorder_set_item.py

@@ -0,0 +1,19 @@
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"material_set": None, "old_index": 0, "new_index": 0}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        if self.settings["material_set"].is_a("IfcMaterialConstituentSet"):
+            set_name = "MaterialConstituents"
+        elif self.settings["material_set"].is_a("IfcMaterialLayerSet"):
+            set_name = "MaterialLayers"
+        elif self.settings["material_set"].is_a("IfcMaterialProfileSet"):
+            set_name = "MaterialProfiles"
+        elif self.settings["material_set"].is_a("IfcMaterialList"):
+            set_name = "Materials"
+        items = list(getattr(self.settings["material_set"], set_name) or [])
+        items.insert(self.settings["new_index"], items.pop(self.settings["old_index"]))
+        setattr(self.settings["material_set"], set_name, items)