Avoid that formatted strings discard units of quantities

[mstimberg]

The big string formatting unification in d3ae592 introduced a regression in a few places, because the default formatting for quantities used in f-strings or .format is to display it as a floating point number, discarding its units:

>>> start = 0*second
>>> duration = 1 * second
>>> print("Starting simulation at t=%s for a duration of %s" % (start, duration))
Starting simulation at t=0. s for a duration of 1. s
>>> print(f"Starting simulation at t={start} for a duration of {duration}")
Starting simulation at t=0.0 for a duration of 1.0

In most places, I worked around this by using something like "... t={start!s}" to manually force a conversion to a string, but I think it would be better if we changed quantities' default for "formatting without format spec" (which would also be more consistent with a simple print(start)). Luckily, we can do this easily using the __format__ function. With this PR, both print statements above give the same output.

[mstimberg]

I'm going ahead with the merge as it should be low risk (and fixes the confusing output in a number of places, e.g. for run(...report='text'), or print(profiling_summary()). It would also be trivial to revert if we find any issue with it.