TMMT: modified Sequences -> SequenceGens in ChainStep; fixed bug from mutable default values
- Within the cleanup campaign of ATR-29039, modified Sequences -> SequenceGens in ChainStep to avoid confusion
- fixed possible bug from mutable default values passed in functions, for all code in TMMT. This was discovered in a precedent MR !74360 (merged), and the solution is discussed also there.