Possibility to have multiple writers

It would be nice to have the possibility to have multiple writers in one algorithm chain. This would make it possible to create e.g. a "debug level" output file and a standard output file. Or further in the future the possibility to run a reco + analysis chain and keep both levels.

Is it already possible now to have multiple `Writer`s? (Including the necessity to sequence it properly)

Start by inspecting the current Writer and algorithm-chain implementation to determine whether multiple writers are supported and how sequencing is handled. Define what must change to support separate debug and standard outputs, or parallel reconstruction and analysis outputs, and document the expected behavior.

Written by the indexing model from the issue text.