Add ability to download more than 2 FastQ files via FTP and Aspera

Description of feature

As raised in https://github.com/nf-core/fetchngs/pull/259#discussion_r1470946683 and https://github.com/nf-core/fetchngs/pull/259#discussion_r1470960788 we need to revisit why we restricted downloading a max of 2 FastQ files via FTP and Aspera.

I vaguely remember this was added because they may have been discrepancies in some files have 3 FastQ files but only 2 md5sum files which broke the pipeline. We need to find some examples of database ids that have 3 FastQ files and take a proper look to see if we can accommodate them in the pipeline.

If you do have more than 2 FastQ files e.g. single-cell data like in this issue https://github.com/nf-core/fetchngs/issues/144 then you should be able to retrieve these by using the --force_sratools_download parameter.

Start by reviewing the linked discussion in pull request 259 and issue 144, then inspect the pipeline's restriction to two FastQ files for FTP and Aspera downloads. Find database IDs with three FastQ files and compare their FastQ and md5sum files. Done means the pipeline can accommodate valid cases with more than two FastQ files without breaking checksum handling.