Add support for indexing protein sequences

[sanjaysrikakulam]

Added support to index amino acid sequences and to create indexes directly from fasta, fastq files and, extracted kmers in a .txt file.

Changes to be made:

1. Update the config YAML files in the docs (https://bigsi.readme.io/docs/your-first-bigsi):
   By default now the code expects to find the following key:value pair in the config YAML file.
   sequence_type: nucleotides
   or
   sequence_type: aminoacids
   I have updated the YAML files found in example-data/configs to adapt this.

[leoisl]

Dear @sanjaysrikakulam ,

I am really sorry for the huge delay on answering you (last weeks were unusual). We are interested in your PR, but could you make the PR at https://github.com/iqbal-lab-org/BIGSI/pulls ? Like this, you will get 2 or 3 people to review it!

Thanks

[sanjaysrikakulam]

Dear @leoisl,

Thank you for the suggestion. I have now created the PR.