Use the new string formatting method in io.ascii.write

As far as I can see the formatsparameter in io.ascii.write still uses the old string formatting.

The new method should be used instead.

[taldcroft]

Takeaway: feel free to change any examples or code from old style (e.g. format='%12.1f') to new style (e.g. 12.1f), which is the preferred idiom. This uses format(val, fmt) to actually do the formatting.

Note that is NOT the same as {:12.1f}, which uses fmt.format(val).

[dhomeier]

There was some recent discussion on performance implications of the different format options in context of logging which might be of interest here as well (the on-demand evaluation probably is not, since everything is written here anyway).