pcl::people::GroundBasedPeopleDetectionApp has missing documentation/small bugs

As these are all relatively minor/easily solved issues, and to save myself some time, I've chosen to file this as a single issue - let me know if this should be broken out though.

There are a few things that make GroundBasedPeopleDetectionApp unusable in some circumstances, and also just very hard to discover how to use in others:

• No documentation on what format it expects the point cloud in. Indeed, it expects x to increase rightwards, y to increase downwards (this was the confusing part) and z to increase outwards (I think - it might not matter).
• It also expects the cloud points to be arranged in a width x height arrangement so that it can later extract a coloured image.
• It would be useful to also document the expected arrangement when 'vertical' is true
• setPersonClusterLimits takes variables called min/max width/height to calculate min/max points for Euclidean clustering, but the algorithm it uses to determine how many points this is isn't documented. It doesn't really correspond to min/max width/height of a human though, despite what the documentation says. This ought to be fleshed out, or perhaps just less overloaded in use (the min/max height does get used as person height later, though it doesn't correspond to height unless that person is standing straight).
• setDimensionLimits is never called on the HeadBasedSubclustering object, so if your cloud happens to be of a configuration where people have more than 5000 points (or fewer than 30), you're out of luck.
• Cluster tolerance is 2 * voxel size, rather than a configurable tolerance or ratio. For the default 6cm voxel size, this is a very high tolerance. It would seem to me that this variable isn't really linked with voxel size.

Start by reading GroundBasedPeopleDetectionApp and HeadBasedSubclustering, focusing on the documented point-cloud assumptions, setPersonClusterLimits, setDimensionLimits, and cluster tolerance. Done would require resolving or documenting the listed cloud layout and parameter behaviors, but the issue bundles several changes and names no tests or files.