RefGenie - build local Nextflow config

[ewels]

Split from #592 - specifically #592 (comment)

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@nsheff managed to add plugin hooks and build an example @nf-core plugin all in a day! Over here: https://github.com/databio/refgenie_nfcore

Can use this to model the RefGenie integration. This will work super nicely because it is so harmless:

• RefGenie installed, nf-core not installed: Nothing
• RefGenie not installed, nf-core installed: Nothing
• RefGenie and nf-core installed: Nextflow genomes config automagically maintained

There will be no action required by users beyond installing two tools that they will be using anyway. 🚀

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So basically, add a setuptools entrypoint to nf-core/tools that executes some custom code within RefGenie runs.

Some slight differences / extensions that I think would be nice compared to the repo above:

• Automatically write to $NXF_HOME/nf-core/genomes.config if path not set otherwise.
• Prompt to add an includeConfig line to the $NXF_HOME/config file if not already there.
  • This means that we can overwrite the genomes.config safely each time.
• Alternatively (skipping the includeConfig thing):
  • Look for an environment variable with a path (NXF_REFGENIE NF_REFGENIE_PATH?)
  • A RefGenie config string

Need to be sure that it fails gracefully if RefGenie is not installed.