bad use of `--overwrite` in measurement script

[Chrisgant]

Using the overwrite flag in

NSL/Executables/Examples/example_k_correlators.cpp

Line 111 in 95a2406

params["overwrite"].to<bool>() ? NSL::File::Truncate : NSL::File::ReadWrite | NSL::File::OpenOrCreate

will delete the configurations, not just the measurements. This behavior is neither intuitive nor useful.