Constructs a loop of all the context-independent phones. This loop is used in the static flat linguist for detecting
out-of-grammar utterances. A 'phoneInsertionProbability' will be added to the score each time a new phone is entered.
To obtain the all-phone search graph loop, simply called the method getSearchGraph.
For futher details of this approach cf. 'Modeling Out-of-vocabulary Words for Robust Speech Recognition', Brazzi,
2000, Proc. ICSLP