better discrimination between stage and step

[MartinPippel]

Is your feature request related to a problem? Please describe.
When starting the pipeline with an already decontaminated assembly you need to specify the steps of the pipeline which should be run and the stage the assembly is already in. I mixed those up and just used the steps for the stages and steps definition.
The pipeline was running even if the stage was not valid - but the given assembly was never picked up for any specified step.

Describe the solution you'd like
It would be nice if the pipeline throughs an error if the specified stage is unknown. Maybe we should add a small paragraph to the documentation which stages do exist.
steps = 'inspect,preprocess,assemble,purge,polish,screen,scaffold,curate'
and
stage = [ raw, decontaminated, polished, scaffolded, curated ]