Setting the pipeline folder to a path that's not absolute causes problem for checkpointing

[vreuter]

When using a Pipeline and configuring the outfolder with a value that's not an absolute path causes the checkpoint call to fail due to trying to write to a file in a subfolder of the same name, e.g. "pypiper_output" --> Path.cwd() / "pypiper_output" / "pypiper_output" / "my_stage.checkpoint" instead of Path.cwd() / "pypiper_output" / "my_stage.checkpoint"