Added IfcPropertySingleValue to EnablePsetEditing to fix bug

Noticed bug trying to edit "Data" in psets from ifc modifiers and fixed it.

Traceback:
Error: Python: Traceback (most recent call last):
  File "\blenderbim\bim\module\pset\operator.py", line 94, in execute
    self.load_from_pset_data(pset)
  File "\blenderbim\bim\module\pset\operator.py", line 206, in load_from_pset_data
    metadata.set_value(value)
UnboundLocalError: local variable 'value' referenced before assignment

src/blenderbim/blenderbim/bim/module/pset/operator.py

@@ -197,7 +197,7 @@ class EnablePsetEditing(bpy.types.Operator):
                     setattr(new, data_type, enum)
                     new.is_selected = enum in selected_enum_items
             else:
-                if prop.is_a("IfcSimplePropertyTemplate"):
+                if prop.is_a() in ("IfcSimplePropertyTemplate", "IfcPropertySingleValue"):
                     value = prop.NominalValue.wrappedValue
                 elif prop.is_a("IfcPhysicalSimpleQuantity"):
                     value = prop[3]