trunk: nnet2 setup: Code and script changes to support dumping multi-frame egs to disk.  See get_egs2.sh, train_pnorm_simple2.sh, etc.

git-svn-id: https://svn.code.sf.net/p/kaldi/code/trunk@4614 5e6a8d80-dfce-4ca6-a32a-6e07a63d50c8