New feature to manipulate cost item quantities

src/ifcopenshell-python/ifcopenshell/api/cost/add_cost_item_quantity.py

@@ -0,0 +1,18 @@
+import ifcopenshell.api
+
+
+class Usecase:
+    def __init__(self, file, **settings):
+        self.file = file
+        self.settings = {"cost_item": None, "ifc_class": "IfcQuantityCount"}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        quantity = self.file.create_entity(self.settings["ifc_class"], Name="Unnamed")
+        quantity[3] = 0.0
+        cost_item = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcCostItem")
+        quantities = list(self.settings["cost_item"].CostQuantities or [])
+        quantities.append(quantity)
+        self.settings["cost_item"].CostQuantities = quantities
+        return quantity