RMF node names for FG roots in FGChain

[barakr]

I omitted the "root" suffix from FG chains in commit fee26c since it renders RMF files within old restart protobuf files incompatible (e.g. in benchmark_full), due to a usage check from rmf/include/atom_links.cpp in add_links_recursive, that expects the old FG chain name to remain the same.

What needs to be done is to add some versioning to the RMF file, or in the restart output file, that says that if an output file is old, the chain names will be in the old form for consistency, and otherwise be changed.