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

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

@@ -0,0 +1,16 @@
+import ifcopenshell
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"constituent_set": None, "material": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        constituents = list(self.settings["constituent_set"].MaterialConstituents or [])
+        constituent = self.file.create_entity("IfcMaterialConstituent", **{"Material": self.settings["material"]})
+        constituents.append(constituent)
+        self.settings["constituent_set"].MaterialConstituents = constituents
+        return constituent