Produce YAML defining production requests instead of using DIRAC directly

LbMCSubmit is starting to write out a machine readable definition of production requests which will soon be ingestable by DIRAC. This will allow a lot of the complicated step/production request code to be removed from LbAnalysisProductions in favour of a simplier interface.

See also: https://gitlab.cern.ch/lhcb-core/lbmcsubmit/-/issues/8