Ambiguous base filter

[esteinig]

Requested by @mbhall88

• [ ] Remove reads with > x % ambiguous bases
• [ ] Command line option for filter and allowed bases
• [ ] Speed optimisation [will have to have a look]
• [ ] Tests

[mbhall88]

In the meantime, Wei Shen is awesome and there is (of course) a way to do this with seqkit https://github.com/shenwei356/seqkit/issues/403

[esteinig]

Love Wei Shen's work - and of course it was available ^^ I'll flag this for the next version.

There should be a fast way of doing this using the &[u8] sequence from a needletail record.