Add passthrough filter

[chambbj]

This could obviously be more complicated, but as it is, I think it's a nice
alternative to the Python filters. The documentation, sample pipeline, and unit
tests should adequately explain the current feature set (basically
multi-dimensional, min/max/exact value criteria for passing points through to
the output buffer).

[chambbj]

Just to clarify, this is not intended to be a full replacement for Python, just a small subset for simple filtering.

[hobu]

Could this be called filters.sieve?

[chambbj]

I have no strong feeling on the name. Passthrough is just the analogue in PCL.

[hobu]

ah, that's fine to keep it aligned then. I think our challenge is that filters itself is probably wrong, since they don't really/always "filter".

[chambbj]

I still have it in the back of my head to possibly split the filters from the non-filters. I struggle a little with what to call the non-filters. I believe @abellgithub proposed "operations" or something similar.

[gadomski]

My two cents: sieve does make more sense than passthrough for me — I had to go look at the source to figure out what passthrough did, but I think I could have intuited sieve.

[chambbj]

Discussed offline -- this basically just filters a range, and we don't have plans to make it any more complicated than that, so call it a range filter.

[chambbj]

I'd missed a file rename on this. Otherwise, assuming it passes Travis, it's all ready to go.