Additional support for germline sets

We have created step-by-step walk-throughs showing how to use the AIRR-C germline sets with local installations of IgBLAST and MiXCR.

These tools make use of download_germline_set, which is a powerful command-line utility which can be used to download sets in various formats, and explore the sets and versions available in OGRDB. It can even create the custom .ndm and .aux files required by IgBLAST.