support for subworkflows

[tiborsimko]

Sometimes your analysis workflow would like to perform other tasks, for example run certain steps to make optimisations. This is not strictly speaking part of "physics analysis", yet needed to trigger from time to time for some runs.

The need could be addressed by allowing "subworkflows", e.g. having central workflow definition and separate "optimisation" workflow definition and trigger for some runs to run both, possibly via "include optimisation.yaml" like commands in the main reana.yaml.

Epic, ticketised for later input.