Allow user adjustment of minimal informative protein threshold

[hoelzer]

Currently, the minimum number of proteins with ViPhOG annotations at each taxonomic level, required for taxonomic assignment, is 2.

However, for certain use cases (see discussion in #64 ) it would be good if the user can change this, e.g. to "1" for short contigs or segmented viruses. However, this might lead to more FP hits.

The parameter is used in this script:

https://github.com/EBI-Metagenomics/emg-viral-pipeline/blob/master/bin/contig_taxonomic_assign.py#L103

which is called in this process:

https://github.com/EBI-Metagenomics/emg-viral-pipeline/blob/master/nextflow/modules/assign.nf#L15