Add option to switch from 'megablast' to 'blastn'

[gregdenay]

This version introduces a new parameter use_blastn, which enables user to switch from the default 'megablast' search for cluster sequence matching to a search using the 'blastn' algorithm. This can help find sequences with lower homology than what is possible with the megablast algorithm.

Configuration files fron the 1.6.x versions can be updated by adding the following line:

use_blastn: False

Checklist

• [x] Title of this PR is meaningful
• [x] Comprehensive description of the changes
• [x] Link eventual issues
• [x] Test modifications
• [x] If there are new parameters: update config template
• [x] Update documentation if nescessary
• [x] Add modifications to changelog.md

[gregdenay]

Not the change that is actually nescessary to fix the low-complexity pb