Pin merge-mates to a tag?

[CBeelen]

We previously just fetched the most up to date merge-mates code from the master branch, but there were some commits recently that broke our code. Specifically, some of the input arguments are no longer supported(--o for the output file, and the --interleave option that we are using in the de novo pipeline).

Currently, the version is pinned to the last commit that still works with our code, 2fec613.

Consider whether we would like to keep updating merge-mates or pin the version permanently - in that case it would be nicer to pin to a tag, either on the original repo or, if we create one, on a fork.