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

src/ifcopenshell-python/ifcopenshell/api/void/add_filling.py

@@ -0,0 +1,16 @@
+import ifcopenshell
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"opening": None, "element": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        self.file.create_entity("IfcRelFillsElement", **{
+            "GlobalId": ifcopenshell.guid.new(),
+            "RelatingOpeningElement": self.settings["opening"],
+            "RelatedBuildingElement": self.settings["element"]
+        })