remove nneval_gpu

remove nneval_gpu
update nneval to take function handle to nnff
update nntrain and nntrain_gpu
update nnmatthew to handle gpu
update nnpredict to handle gpu

nnpredict now takes a handle to the forward function which should be
either nnff or nnff_gpu

nntest update to handle this
example update to handle new nntest