Additional support for prediction

[piotrszul]

This is the remaining work from issue: #140

That is:

• Add command line option for predicing class probabilities
• Implementing command line predictions form JSON serialised model
• Adding option for checking if the features in the prediction data match the model
• Implementing the same functionality in Hail API
• Implementing the same functionality in Python API
• Add support for ONYX

[BMJHayward]

Thanks Piotr. You've been just blasting through work on variantspark lately - blowing up my email. Currently I'm still looking at JSON deserialisation, see branch https://github.com/BMJHayward/VariantSpark/tree/170_loadingJSONmodel on my fork.