Even if the attribute is single, we should be able to force a call to get an array of values.

In some cases, we are not sure the value is single or multiple, for example the attribute domain in the ODF is defined as multiple but is sometimes overrided as non-multiple.

Maybe, makes the cardinality overridable should be forbidden, we should have a widget with max selected values and the value could be stored as an array even if in the project (like odf-demo) the attribute is overrided to select only one value.