amptk v0.9.1

• Add phix filtering for Illumina data. As part of the PE merging function in amptk illumina and amptk illumina2, scripts will also now run phix removal.
• Workaround for DADA2 error where samples that only have 1 read post filtering trigger a derep$quals matrix error. amptk dada2 now has -m, --min_reads option to drop samples that have fewer than -m reads. Default this is set to 10, however, in practice probably this should be much higher, but this should avoid the above error.