Support Local Singularity Images

[cmarkello]

A specific use-case for the NIH's Biowulf cluster where support for large-enough disk space for singularity runtime containers is needed for generating whole-genome graph reference indexes.

Currently the URL's specified for the containers in the config file do not have any prefix or marker to determine whether to treat a container URL as a local file URL or a URL to a remote repository. I propose adding a check for a prefix of "file://" for the given container tool URL to the singularity container calling functions src/toil_vg/singularity.py to support this feature.