New module: xengsort/classify

[LeonHornich]

Added new module xengsort/classify for classifying reads using a existing index file.

Notes:

• apart from versions_xengsort, all output files are declared optional. This is due to the optional parameters --filter and --count. When specifying those a different set of files is produced.
• in the nf-tests i only check for file creation since i got md5sum for empty file found error otherwise. I am assuming this is due to the minimum and small nature of the testing data

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[SPPearce]

Are all your output files unstable? That doesn't seem great for the tool...
Can you use sanitizeOutput from nft-utils please.

[SPPearce]

Why are all the output files unstable?