Distiller updates

[agalitsyna]

A couple of points on distiller that might improve the user experience with it:

1. Distiller needs docs. Parameters of the project.yml file can be described in nice documentation instead of comment lines.
2. There is a special tool for building the docs from the JSON schema in nf-core. It might be used without complying with other nf-core rules. Do we probably want to try it?
3. @Phlya pointed to the lengthy commands in the distiller processes, and it might be solved by nextflow's stdout output of the channels.
4. We might consider transferring to DSL2, but first, we probably want to discuss the modules/subworkflows/workflows structure.

[golobor]

+1 to all suggestions - docs/compliance with nf-core standards/DSL2!