Mdl/fgcsbench

• added .qs2 from CRAN package qs2 as new default format. Options --rds and --qs (also -r and -q respectively in the initializer) allow to choose one of those formats
• rationalized a bit the messages to stdout both in the R functions and in CPP

In poet.cpp:

• commented out using std;
• added #include <format> in order to use std::format("{:.2f}", dt)