fix LAS export scaling issues
fix LAS export scaling issues

Set a scale factor on the LAS header so that points have millimeter
rather than meter precision. Pass the header when constructing points so
that it knows how to scale the input coordinates for proper export.
Properly scale the time values from microseconds to seconds.

With these changes, the LAS output should be "reasonable" (with the
caveat that the position data is still sensor-relative, and the time
epoch seems to be something unusual).

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