[PATCH] improve Interval; make 'qinterval' accurate to documentation

[Qian Yun]

In Interval, now 'qinterval' is accurate to documentation;
some code is duplicated from AIntervalCategory, which was
moved to there before, but since Interval is for FPS which
is inaccurate, so the rounding is inevitable and the code
has to be duplicated.

Also slightly improves accuracy for 'abs' and '* for ZZ'.

- Qian