A unit state that modifies how the unit state is cached. Caching keys are generated from the full name for the
sentence hmm. The default behavior for the unit (and all sentence hmms) is to generate the full name by combining the
name for this unit with the name of the parent. For the simple linguist, this is undesirable, because there are many
different names for the parent pronunciations (differing contexts). We want to be able to combine units that have
identical names and context and are in the same position in the same pronunciation. By defining getFullName to
combine the name and the pronunciation index we allow units with identical contexts in the same position in a
pronunciation to be combined.