Returns the integer double value closest to
Rounds away from zero when there is no closest integer:
(3.5).roundToDouble() == 4 and
(-3.5).roundToDouble() == -4.
If this is already an integer valued double, including
-0.0, or it is not
a finite value, the value is returned unmodified.
For the purpose of rounding,
-0.0 is considered to be below
-0.0 is therefore considered closer to negative numbers than
This means that for a value,
d in the range
-0.5 < d < 0.0,
the result is