update: modularized gbnn_messages2thrifty.m so that users can easily convert full messages into sparse thrifty messages + automatic full messages to sparse thrifty messages conversion in gbnn_test.m and gbnn_correct.m (you can use the same messages matrix for gbnn_learn.m and test now) + renamed sparsemessages -> thriftymessages and added comments (the difference with Chi sparse cliques and thrifty codes should be more clear)

Signed-off-by: Stephen L. <lrq3000@gmail.com>