Add support for single fastq

[dfornika]

The downsampling tool we're using in this pipeline (rasusa) has support for downsampling from a single fastq file. That would be useful for downsampling long-read nanopore libraries, which only produce a single fastq file per sample.

The pipeline currently only supports downsampling paired fastq files, such as those produced by illumina paired-end runs.

Add support for downsampling from a single fastq file.