Document laser scanner, including tranformation

[dlebauer]

Equation (from Zongyang):

X: ply coordinate, X’: gantry coordinate, unit: mm
x_offset: origin offset
Gx: gantry position report in metadata

X’ = X + (Gx1000 + x_offset), x_offset = 82
Z’ = Z + (Gz1000 + Z_HIGHER) - z_offset, Z_HIGHER represent for gantry origin is 30 cm higher than the field, so Z_HIGHER = 300, z_offset = 3445

equation Y is different in positive scan and negative scan, in positive scan:

Y’ = Y + y_offset_positive, y_offset_positive = 3450
Y’ = Y + y_offset_negative, y_offset_negative = 25771

related: terraref/computing-pipeline#363 (comment)
terraref/reference-data#44
implementation: https://github.com/terraref/extractors-3dscanner/blob/master/ply2las/ply2las.py#L78