Fixed bug in passthrough filter

- Points on filter boundaries were included into inliers (i.e.
  'setFilterLimitsNegative (false)') and outliers (i.e.
  'setFilterLimitsNegative (true)'). Now points on the boundaries belong
  only to the set of inliers.
Merge pull request #331 from SvenAlbrecht/bugfix_passthrough

Fixed bug in passthrough filter