FASTQ average quality score calculation can be faster

[rhpvorderman]

See nanoporetech/bonito#336

Also thanks for your post on https://gigabaseorgigabyte.wordpress.com/2017/06/26/averaging-basecall-quality-scores-the-right-way/ it was tremendously helpful.
Unfortunately a lot of tools still do it wrong. Even FastQC(!).

[wdecoster]

Isn't what you suggest implemented here?
I believe the function errs_tab() is executed only once when ave_qual() is initialized (credits go to @bsipos for wdecoster/nanomath#13).

[rhpvorderman]

Whoops. Looks like I completely misread your code. In that case, I am sorry to have bothered you!