Update duotone time to higher resolution

DUOTONE_TIME_FLOAT now provides a floating point version of DUOTONE_TIME. DUOTONE_TIME remains unchanged.

GDSTP and IO detail screens now use DUOTONE_TIME_FLOAT. The value is trendable in NDS. The lower limit for duotone time is dropped from 5 to 4 usec, since in our one-sample test, the time averaged close to 5.1 and barely stayed above 5 with noise.

assigned to @ezekiel.dohmen

merged

mentioned in commit 51fd0a23

mentioned in issue #327 (closed)