Add an nf-core module to pipeline (exisiting or create a new one with ncbi_download)

[SimonDMurray]

Use https://github.com/nf-core/modules/blob/master/modules/nf-core/ncbigenomedownload/main.nf as our example nf core integration of a module

[SimonDMurray]

add a "local" and "nf-core" subdirectory to the modules folder and then add the nf-core ncbidownload module and have a parameter that toggles between the two:
https://github.com/nf-core/modules/blob/master/modules/nf-core/ncbigenomedownload/main.nf

[SimonDMurray]

Added fastavalidator and seqkit-stats nf-core modules to pipeline