Add PassThrough for index extraction

This module behaves similarly to PassThrough with two changes:

- It also accepts an optional set of input indices of the input cloud
  It's probably not good to add this to PassThrough as well, because it breaks whatever[:] >> passthrough[:]

- It produces PointIndices of the original cloud instead of a new cloud
Merge pull request #34 from v4hn/pass-through-indices

Add PassThrough for index extraction