Support gzipped barcode files

[Mirochill]

Summary

• Read barcode lists with gzip.open(..., rt) when the file path ends in .gz.
• Normalize gzipped barcode lists into the existing temporary .txt barcode chunks so downstream scripts can keep using normal text files.
• Add regression coverage for counting and splitting gzipped barcode files.

Fixes #106.

Validation

• git diff --check
• git diff --cached --check
• Not run locally: test suite