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

src/ifcopenshell-python/ifcopenshell/api/structural/add_structural_boundary_condition.py

@@ -0,0 +1,15 @@
+class Usecase:
+    def __init__(self, file, settings={}):
+        self.file = file
+        self.settings = {"connection": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        if self.settings["connection"].is_a("IfcStructuralPointConnection"):
+            boundary_class = "IfcBoundaryNodeCondition"
+        elif self.settings["connection"].is_a("IfcStructuralCurveConnection"):
+            boundary_class = "IfcBoundaryEdgeCondition"
+        elif self.settings["connection"].is_a("IfcStructuralSurfaceConnection"):
+            boundary_class = "IfcBoundaryFaceCondition"
+        self.settings["connection"].AppliedCondition = self.file.create_entity(boundary_class)