covmed renamed to covstats in goleft

[martiliasf]

https://github.com/brentp/goleft/blob/1137ebe531e67d0d081cdb1900d1ec065501f246/HISTORY.md

[hdashnow]

Unfortunately the latest version of goleft fails in this context due to very low numbers of reads (fair enough). For the moment I'm sticking with version 1.8 while I explore other options.