add point cloud attributes to segment statistics

[iosefa]

Current statistics or object creation include those from spectral and textural properties. It will be beneficial to be able to include structural properties for classification. These three at least should be pretty low hanging fruit:

• height
• PAI
• LAD
• FHD

We can also include lidar specific properties like intensity.

For these, I propose that we allow two options:

1. to calculate a single value for the entire segment
2. to calculate statistics in grid cells and return some aggregate; for example, mean and variation.