sortreads: add option to make anchored/non-anchored search when searching for tags (demultiplexing)

[meglecz]

Demultiplexing should be anchored by default, but we should give the possibility to users to make it non-anchored. The non-anchored search is slower, but it is necessary if there is a spacer before the tag.
The present version is a non-anchored search.

The choice of anchored/non-anchored search should be independent for demultiplexing and for trimming primers.

Format of barcodes.fasta without anchored search :

>marker-run-sample-replicate
tcgatcacgatgt;min_overlap=length-of-the-tag...gctgtagatcgaca;min_overlap=length-of-the-tag