对有类别监督的数据进行先行判别分析用于降维

数据为结构体类型，返回数据也是降维后的结构体类型。如(a.X a.y)a.X是带有类别的数据，a.y是类别信息-Synopsis:

　 model = lda(data)

　 model = lda(data,new_dim)



　Descr iption:

　 This function is implementation of Linear Discriminant Analysis.

　 The goal is to train the linear transform which maximizes ratio

　 between between-class and within-class scatter matrix of projected

　 data.