Adjust avg and stddev computation and printing

[jmid]

This PR

• adjusts the avg and stddev computations to limit needless precision loss. It does so by adding summands by order of magnitude (smaller to larger),
• it switches printing of avg and stddev to scientific notation once we go beyond 1e7, from the rationale that, e.g., stddev: 2660730801206827008.00 contains more decimals than are readable and that we have precision for,
• it works around an annoying MinGW printing issue that inserts a leading exponent zero as in "1.859e+018" which needlessly caused expect tests to fail there, and
• finally it reenables dune runtest on Windows

Fixes #293

[jmid]

Rebased on main - and also fixed a bug in the 'remove-leading-zeros-from-float-exponent-for-consistent-printing' function cut_exp_zero.