output/csv: Avoid accumulated timestamp error for odd samplerates

output/csv: Avoid accumulated timestamp error for odd samplerates

The conversion of sample rates to sample periods and the repeated
addition of truncated values (integer variables) resulted in the
accumulation of errors in the timestamp column for odd samplerate
values. How to reproduce:

  $ sigrok-cli -d demo:analog_channels=0 \
      -c samplerate=6000000 --samples 1200001 \
      -O csv:time=true | tail

Accept the additional cost to reduce the error. Always get the timestamp
in a new calculation based on the sample number and the sample rate.

This addresses bug #1027.

Signed-off-by: Earle F. Philhower, III <redacted>
[ gsi: rephrased commit message, how to reprodue ]