Some tweaks on top of the restructure_outputs branch.

[mberacochea]

Changes:

• Simplify the assembler selection (SE -> megahit, PE -> metaspades)
• Migrate from nf-validation to nf-schema
• nf-core lint fixes (they won't work now as nf-core tools doesn't fully support nf-schema)
• nf-test using the test data (the same used for `nextflow run main.nf -profile test,<docker,singularity>)
• Moved the publishDir directives to the modules.conf file
• Added a method to generate the study_prefix/study/reads_prefix/reads folder

[KateSakharova]

very nice!