container image caching (like `renee cache`) for nextflow & snakemake pipelines

[kelly-sovacool]

Implement similar logic as renee cache and image_cache() for nextflow pipelines, so a shared directory of SIFs can be used.

groovy config

python code for reading groovy conf/containers.config files: https://stackoverflow.com/questions/32216707/parsing-a-groovy-file-in-python

or quick way with bash:
cat conf/containers.config | grep '=' | sed 's/.*= /docker:\/\//g' | sed "s/'//g"