Borrow ideas for how to design "restart" feature from sarek

[jmuhlich]

Description of feature

Sarek has some nice logic to restart a previous run from a specific step in the pipeline. At each restart point in the pipeline, they emit a samplesheet into outdir that represents the state of the pipeline, potentially referencing other files in outdir. When a later run specifies the step param, the relevant samplesheet is automatically pulled out of outdir. This seems like a good way to implement the old mcmicro start-at feature while still retaining the separation of inputs and outdir.