How to indicate online processing capabilities?

[superbock]

At the moment there is no way to determine whether a processor is online-capable or not.

A previous attempt to solve this used different process methods. E.g. process_sequence() and process_step() for offline/online mode, respectively. However, since normal functions must be usable as processors also, Processors are simply called (like functions), thus there is not really a uniform way to distinguish between online/offline mode for processors.

Please see this PR/comment for more context.