Training algorithms other than lBFGS are not supported for semi-Markov and higher-order models

Example:

> ./frontend/crfsuite learn --type=semim -m semim.model -a l2sgd tests/test_sm_1.input 
> CRFSuite 0.12  Copyright (c) 2007-2011 Naoaki Okazaki

ERROR: Training algorithm 'l2sgd' is not supported for this type of graphical model.  Try `lbfgs' instead.
