Optimise newick tree generation

Using a bit of C++11 magic (std::to_string), I was able to significantly reduce the time for generating newick trees:

    # Old:
    time scrm 20 1 -r 4000 10000000 -l 300000 -seed 17 -T > /dev/null                                                                                                                   
    real    0m1.770s
    user    0m1.745s
    sys     0m0.020s

    # With optimization:
    time ./scrm 20 1 -r 4000 10000000 -l 300000 -seed 17 -T > /dev/null
    real    0m1.100s
    user    0m1.090s
    sys     0m0.008s