.sha256 in addition to 'normal' file hashes

We new support jobs creating
.sha256 files containing 64 char hexdigest (presumably sha256 hashes)
next to their output files.

These are then used instead of recalculating a hash using xxhash_128().

This allows for both speed ups (for example tools like mbf_fastq_parser
can generate such hash files essentially for free),
and for file hashes that are not strictly a hash of the bytes.

(For example the STAR aligner will generate different compressed files,
that are nevertheless identical in uncompressed content).