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

src/ifcopenshell-python/ifcopenshell/api/pset_template/add_prop_template.py

@@ -0,0 +1,22 @@
+import ifcopenshell
+
+
+class Usecase:
+    def __init__(self, file, settings={}):
+        self.file = file
+        self.settings = {"pset_template": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        prop_template = self.file.create_entity("IfcSimplePropertyTemplate", **{
+            "GlobalId": ifcopenshell.guid.new(),
+            "Name": "NewProperty",
+            "PrimaryMeasureType": "IfcLabel",
+            "TemplateType": "P_SINGLEVALUE",
+            "AccessState": "READWRITE"
+        })
+        has_property_templates = list(self.settings["pset_template"].HasPropertyTemplates or [])
+        has_property_templates.append(prop_template)
+        self.settings["pset_template"].HasPropertyTemplates = has_property_templates
+        return prop_template