Request: `singularity` dependency for detecing that singularity-based planners can run

# Context

- Some users might want to use planutils in Docker but with no privileged access
- Right now, only a few packages works, including VAL and pyperplan

# Definition of done

- Installing "singularity" package check if singularity planners can run, reporting the need for privileged access
- Installing planners that use singularity would fail right away if
- Documentation specifies that singularity is needed for most plans, and that installing them fails quickly

# Implementation idea

- “singularity” install pulls a tiny singularity image and attempt to run it
- first installation attempt use some bandwidth
- new calls would be faster because image is in singularity cache

# Checklist

* [ ] Should check to see if it's installed and has the right version/privileges required to run the planners that need it.
* [ ] Warn the user (fail to install) if it's not setup right
* [ ] Planners that use singularity should have this package as a dependency