PORT: the MM 1.4 diSPIM plugin uses logical negation for this check

[bls337]

This is a fix for an issue identified in #421

We need to use !acqSettings_.isUsingMultiplePositions() here to match the original plugin.

      // make sure we aren't trying to collect timepoints faster than we can
      if (!acqSettings.useMultiPositions && acqSettings.numTimepoints > 1) {
         if (timepointIntervalMs < volumeDuration) {
            MyDialogUtils.showError("Time point interval shorter than" +
                  " the time to collect a single volume.", hideErrors);
            return AcquisitionStatus.FATAL_ERROR;
         }
      }