Align TimeWithUnit printing to the "normal" double behaviour

This merge substitutes !419 (closed)

With respect to the old one, it does change how time is printed, only when using .As (Time::Unit).

The result is like this:

 0.0067  -0.0017   0.0017   1.1   10   1e+02
0.0067s -0.0017s 0.0017s 1.1s 10s 1e+02s
+0.00665999999999999996 -0.00166599999999999995 +0.00166599899999999998 +1.11099999899999999996 +10.11099999999999999998 +100.11099999999999999998

Where the first line is double printing, the second is time printed with .As (Time::Unit) and the 3rd is a time printed with .To (Time::Unit).

The latter (.To (Time::Unit)) is useful when the time must be printed without loss of precision (at the expense of extra numbers).

time-print.cc