Create and upload singularity container of treecript for each release

[jmfernandez]

A way to distribute treecript with all its dependencies is through a singularity image, so it can be used in isolated environments, like HPC ones. It is possible to run treecript from within a singularity/apptainer instance, if and only if the instance is not run in an isolated PID namespace (because there it would not be able to gather metrics from other processes running outside).

There is a GitHub CI workflow which allows building a singularity image => https://github.com/singularityhub/github-ci .

This issue should be fulfilled when:

• A singularity recipe for treecript is created.
• A Github CI workflow is setup to create and upload the singularity container images to the GHCR .
• Document how to fetch and use the treecript singularity in an HPC environment.