add var barcodecop_head_cmd and append to barcodecop processes

[dhoogest]

Does not implement downsampling when index = none, since that condition does not run barcodcop.

@nhoffman opening this for discussion primarily

[dhoogest]

@crosenth suggestion is to add a --head arg to read_counts.py and use fastalite functionality to downsample in addition to generating counts, for the conditional where barcodcop isn't used

[crosenth]

@dhoogest - https://github.com/nhoffman/dada2-nf/pull/45/commits/99843967d5e976658aa4d6a728e4f50639d0a454

[dhoogest]

Thanks @crosenth - looks good to me. @nhoffman plz review and merge at your convenience.