Use of singularity requires pulling of docker image before running, but would be best if nextflow automatically did this

[srcrowl]

In order to use the nextflow implementation with Singularity, you currently have to pull from dockerhub manually before running and save the .sif file in the nextflow repository. Ideally, this would happen automatically and not require users to input the command themselves. 2 things:

1. Include in documentation for using singularity to pull from docker image

singularity pull docker://naeglelab/kstar-nextflow:latest

2. Figure out how to automatically do this. Once this is done, remove (1) from documentation