add json-based config for transport and digitization

Implement text-based config of transport and digitization as a first part of the full simulation chain configuration (see https://redmine.cbm.gsi.de/issues/1970).

mentioned in merge request !297 (closed)

Dear @f.uhlig, @v.friese, @p.-a.loizeau,

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• de59ff48 - add tests + small edits

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