better image management

Sometimes images do not download. Perhaps it would be useful to download them to a directory that remains, so that they are only downloaded if they are not already present? One possibility might be to use the Singularity cache environment variable. WARN: Singularity cache directory has not been defined -- Remote image will be stored in the path: /work/user/choede/testMetagWGS2.5.0/test3genomes/output_coassembly/work/singularity -- Use the environment variable NXF_SINGULARITY_CACHEDIR to specify a different location