Check if input sequence identifiers follow the PanSN-Spec

[subwaystation]

Description of feature

Add code to check if the sequence identifiers of the input FASTA follow the PanSN-Spec as implemented in https://github.com/pangenome/pggb/pull/321/files. If a sequence does not follow the Spec, one should print a warning to the actual Nextflow console!

[heuermh]

Do we have to scan the input FASTA format for this, or can we just look at an index?

[subwaystation]

An .fai would be sufficient here! If I have the workflow correctly in my mind, we ensure we have one at any time. This sounds like a smart way forward @heuermh.