New assign / unassign declaration usecase, minor code & formatting cleanup

src/ifcopenshell-python/ifcopenshell/api/sequence/edit_work_time.py

@@ -0,0 +1,15 @@
+import ifcopenshell.util.date
+
+
+class Usecase:
+    def __init__(self, file, **settings):
+        self.file = file
+        self.settings = {"work_time": None, "attributes": {}}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        for name, value in self.settings["attributes"].items():
+            if name in ["Start", "Finish"]:
+                value = ifcopenshell.util.date.datetime2ifc(value, "IfcDate")
+            setattr(self.settings["work_time"], name, value)