add function msf_delta#1

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ybdarrenwang wants to merge 3 commits intojameslyons:masterfrom

ybdarrenwang:master

Commits on Oct 5, 2016

changed preemph default to 0.97 and added explanation in msf_mfcc; added function msf_delta for delta feature calculation
ybdarrenwang
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Commits on Oct 28, 2016

removed preemph change, focus on msf_delta
ybdarrenwang
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further removed preemph change
ybdarrenwang
committed