Thanks for the explanation, Robert Kern. It seems then that indeed, these days
the optimal default would be `fmt='%.16e'`.
The test on StackOverflow was just a quick demonstration. See also the
description for `DBL_DECIMAL_DIG` here:
https://en.cppreference.com/w/cpp/header/cfloat
> But if your
Double-precision numbers need at most 17 significant decimal digits to be
serialised losslessly. Yet, savetxt() uses 19 by default, meaning that most
files produced with savetxt() takes up about 9% more disk space than they need
to, without any benefit. I have described the problem more detailed
