Update openptv fork with recent features

[yosefm]

The following features were added on my fork and with this PR will be added to the openptv fork:

• Spatial derivatives by deriving the interpolator. Includes a move to an interpolator class that records some scene data for reduced recalculation.
• Add collect() to single-partice scene objects.
• Some tweaks to SG smoothing and other odds and ends.