Draft: After the tools moved to unique_ptr. Try to simplify the ownership in the extrapolator

The propagators moved to returning unique_ptr.

• In principle this should mean we do not suffer from aliasing.
• Try to replace ObjContainer with a simpler implementation. But retaining the same interface for most part.
• Try to make more obvious that is the Cache that holds the vector of unique_ptr
• Remove a "bunch" of release() . This is the kind of more obvious change in the .cxx