Added enroot support.

enroot is another fully unprivileged container runtime.

New options:

• -E option for converting to enroot squashfs image
• --enroot_file for specifying enroot filename

cc: @TobiasMeisel In this MR I also changed the ogs install location to /usr/local/ogs. Also the ogs binaries are now always in the PATH, no need to specify --app ogs on singularity exec-calls. I will updated the ogs docs accordingly.