Define completion sequences
Completion sequences are defined (as arrival sequences) from an arrival sequence and a schedule.
This follows the suggestion from @bbb in https://gitlab.mpi-sws.org/RT-PROOFS/project-administration/-/issues/23#note_55292
This may already exist in a formalization of CPA, ccing @mlesourd and @sophie who may have an opinion here.
Edited by Pierre Roux