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

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

@@ -0,0 +1,19 @@
+import ifcopenshell
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"layer_set": None, "material": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        layers = list(self.settings["layer_set"].MaterialLayers or [])
+        layer = self.file.create_entity("IfcMaterialLayer", **{
+            "Material": self.settings["material"],
+            "LayerThickness": 0.
+        })
+        layers.append(layer)
+        self.settings["layer_set"].MaterialLayers = layers
+        return layer