Revision of Singularity Python for V2.5

[vsoch]

This will revamp the now deprecated Singularity Python to (finally) completely address #81 Specifically:

• singularity-cli: (spython) is now the basic Singularity client, a wrapper to Singularity that also supports recipe conversion
• Global Client (sregistry) is the local management tool for interaction with APIs
• This library is just the more scientific / advanced functions for Singularity images, specifically for metrics / comparison etc.

Now that the clients have been cleanly separated, we can focus more on some scientific and research components of Singularity containers that are of interest to the community, in continuation of this work.

This update will also coincide with the version of the library being used by the current builder on Singularity Hub, and will be merged when the transition (and testing) for v2.5 builders is complete.

[vsoch]

This has been merged with fixes in #92