v2.4.0

In this release:

• #698: sanitize container names.
• #699: transparent execution of actions in Singularity.
• #703: on CI executions, allow ability to pass any popper run flag via commit message
• #706: warn of unsupported features in Singularity
• #708: add CI steps for installing Singularity
• #711: drop --recursive flag
• #712: ability to inject pre- and post- workflows
• #724: remove popper add
• unit testing in Python (#719, #722)
• several bug fixes (#705, #718, #725).

Many thanks to @JayjeetAtGithub, @arshul and @malept for their contributions!