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

src/ifcopenshell-python/ifcopenshell/api/layer/data.py

@@ -0,0 +1,35 @@
+import ifcopenshell
+import ifcopenshell.util.date
+from datetime import datetime
+
+
+class Data:
+    is_loaded = False
+    items = {}
+    layers = {}
+
+    @classmethod
+    def purge(cls):
+        cls.is_loaded = False
+        cls.items = {}
+        cls.layers = {}
+
+    @classmethod
+    def load(cls, file):
+        cls._file = file
+        if not cls._file:
+            return
+        cls.load_layers()
+        cls.is_loaded = True
+
+    @classmethod
+    def load_layers(cls):
+        cls.layers = {}
+        cls.items = {}
+        for layer in cls._file.by_type("IfcPresentationLayerAssignment"):
+            data = layer.get_info()
+            if layer.AssignedItems:
+                for item in layer.AssignedItems:
+                    cls.items.setdefault(item.id(), []).append(layer.id())
+            del data["AssignedItems"]
+            cls.layers[layer.id()] = data