Implement snpeff annotation, snp-df nb and process nbs

[sanjaynagi]

This PR will resolve #80 #81 and #71.

1. We introduce Snpeff for variant annotation. db is specified in the config.
2. We implement a notebook, snp-dataframe, which converts vcf to dataframe and .xlsx
3. We add a notebook, process-notebooks, which adds a cell tag to each nb, so that papermills injected parameters do not show up in the results-book.

@ChabbyTMD Please take a look when you can. I've also made some changes over the last day or two, adding whole amplicon variant calling #78 and a welcome message when starting the workflow #76 .

[ChabbyTMD]

Quite a great implementation of snpeff and the other features . I'll test them on my end and let you know if everything is all good.