Popper library and REANA workflow engine

The current way of interacting with the Popper engine is through the Popper CLI tool. This tool implements all the features of the engine including execution, scaffolding, CI integration, etc. A project that wishes to use the Popper engine has to use the CLI tool. I propose separating Popper into two parts in order to make it easier to use the workflow execution engine in other projects: First a library with all the execution engine itself. Second, a CLI frontend that implements the aforementioned library. With these changes users will be able to extend Popper by creating new ActionRunner and Workflow subclasses. In addition I wish to implement a REANA workflow engine for Popper, this will allow Popper workflows to use all the features REANNA has like kubernetes, workflow specifications, shared storage systems, container technologies, etc. This will also expand the REANA ecosystem and give existing users of the platform another way of defining workflows.