SequencesLoader: implemented a product factory

Several code paths were tested.

Key:
[x] = Pass
[!] = Fail
[-] = Not applicable
[?] = Not evaluated
[ ] = Manual review needed

[x] fasta
[x] fasta.gz
[x] fasta.bz2
[x] fastq
[x] fastq.gz
[x] fastq.bz2
[x] export.txt
[x] sff

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
Mock: updated documentation for new export format

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>