BF + TST: Reinstated eig_from_lo_tri

Maybe someone can tell me what the use-case is for this function? It used to be here as an input to the Tensor model, but that structure was changed long ago, so I am not sure what you would use this for... Anyway - it's back, by popular demand: 

http://mail.scipy.org/pipermail/nipy-devel/2014-December/010734.html