- perform link checks *before* downloading the data

  With the previous version it could happen that you waited a long time
  for the data to download, just to learn at the end that you had an
  unequal number of fw and rv datasets.

- treat lists of links that share the same PE indicator as SE

  This enables working with SE links that end in _1.fastq.gz as found in SRA
  submissions.

- modularize the check of links for easier testing of the code (t.b.d.)