implement point format 6, 7, and 8 (only) support (#72)

* implement point format 6, 7, and 8 (only) support

* extents and pointformat support. extra bytes not quite working yet

* bad operator

* support extra bytes, use 1.4 return counts

* clean up gathering stats for eb

* add back XYZ stats to reflect current extents VLR language

* Add lock for grabbing chunk location information.
Close #76

* updates as suggested

* clean up CopcExtent stuff a bit

* get rid of ctor

* catch up to current copc.io draft

* preserve position seeking behavior from before

* put back root_hier_offset and root_hier_size

* Don't write chunks when there are no points.
Close #80

* updates in response to comments

* update as suggested

* put back previous behavior of FileDimInfo additions

* oops, use the right dimension order

Co-authored-by: Andrew Bell <andrew.bell.ia@gmail.com>