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

src/ifcopenshell-python/ifcopenshell/api/context/add_context.py

@@ -0,0 +1,45 @@
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "context": None,
+            "subcontext": None,
+            "target_view": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        parent = [
+            c
+            for c in self.file.by_type("IfcGeometricRepresentationContext")
+            if c.ContextType == self.settings["context"]
+        ]
+        if not parent:
+            self.create_origin()
+            if self.settings["context"] == "Plan":
+                context = self.file.createIfcGeometricRepresentationContext(None, "Plan", 2, 1.0e-05, self.origin)
+            else:
+                context = self.file.createIfcGeometricRepresentationContext(None, "Model", 3, 1.0e-05, self.origin)
+            project = self.file.by_type("IfcProject")[0]
+            if project.RepresentationContexts:
+                contexts = list(project.RepresentationContexts)
+            else:
+                contexts = []
+            contexts.append(context)
+            project.RepresentationContexts = contexts
+            return context
+        parent = parent[0]
+        return self.file.create_entity("IfcGeometricRepresentationSubContext", **{
+            "ContextIdentifier": self.settings["subcontext"],
+            "ContextType": self.settings["context"],
+            "ParentContext": parent,
+            "TargetView": self.settings["target_view"],
+        })
+
+    def create_origin(self):
+        self.origin = self.file.createIfcAxis2Placement3D(
+            self.file.createIfcCartesianPoint((0.0, 0.0, 0.0)),
+            self.file.createIfcDirection((0.0, 0.0, 1.0)),
+            self.file.createIfcDirection((1.0, 0.0, 0.0)),
+        )