App_common: add `flush_result_transform`
app.transform: draft implementation of #100
Fastq: add Transform.fasta_pair_to_fastq (#100)
app.transform: use Fastq's `fasta_pair_to_fastq`

This is a clean-up related to #100.
App_common: add `two_files_to_file`
App_transform: simplify with `two_files_to_file`
App_common: fix `two_files_to_file` (close/errors)

With this patch the function closes the channels even when there has
been an error in the loop.
app.transform: update manual (#100 and `Fasta`)