Merge the code base of SingSquashfsDataProvider and SingBindmountDataProvider

[prioux]

SingSquashfsDataProvider and SingBindmountDataProvider are both data providers that fetch file data from a Singularity container. The difference between them is that:

• SingSquashfsDataProvider's container is launched with one or several --overlay data.sqs options
• SingBindmountDataProvider's container is launched with a single -B data.ext3:/cbrain:image-src=/,ro option (where /cbrain and / are hardcoded but could be made more general too)

I think we could write a more generic data provider class that implements both these functions. It could support one or many overlays and bind mounts, simulataneously

We need to figure out where to put all the configuration information (including location of the container image, the data files, and the mount path).