added helper: calculateBounds(PointBufferSet)
fixes and improvements to Bounds

- use “lowest” instead of “min”
- make calcBounds() loop simpler
- unittestage
Merge pull request #518 from PDAL/pbset-bounds

added helper: calculateBounds(PointBufferSet)
Merge branch 'master' of https://github.com/PDAL/PDAL into las1.4