regression.py: don't round in compute average for integers

Signed-off-by: Amos Kong <akong@redhat.com>
regression.py: adjust round bit in calculating rate

Display 4 decimal digits of the rate is less than 100,
display 2 decimal digits of the rate is more than 100.

Signed-off-by: Amos Kong <akong@redhat.com>
regression.py: don't round in middle stage of calculations

For improving the precision, we don't round in the middle stage of
calculations, we just use rounding for the display.

Signed-off-by: Amos Kong <akong@redhat.com>
regression.py: do rounding in display stage

We don't round in middle stage of calculations, this patch
do rounding in display stage.

Display 4 decimal digits of the value is less than 100,
display 2 decimal digits of the value is more than 100.

Signed-off-by: Amos Kong <akong@redhat.com>
regression.py: still display Significance line that contains 'nan' value

T-test might returns 'nan' p-value, Significance line will wrongly miss.

Signed-off-by: Amos Kong <akong@redhat.com>
Merge pull request #1516 from kongove/perf-rounding

regression.py: improve result precision by fixing rounding