chord model v2

[bmcfee]

To-dos:

• [x] #10 by adding contributed keras layer classes; squeezelayer
• [x] re-train with HCQT using h=[1,2,3] to reduce root->5th errors
• [ ] re-train with the jazz corpus https://mtg.github.io/JAAH/
• [x] upgrade decoder to use viterbi with spike&slab transitions

[bmcfee]

Done except for retraining with jazz corpus.