Document new _PCL types and usage

[dehann]

Initial example, see here:

Caesar.jl/examples/marine/asv/philos/IngestROSbag.jl

Lines 137 to 186 in 1f7d843

	function handleRadarPointcloud!(msg::sensor_msgs.msg.PointCloud2, fg::AbstractDFG, systemstate::SystemState)
	@info "handleRadarPointcloud" maxlog=10
	# assume there is still space (previously cleared)
	# add new piece of radar point cloud to queue for later processing.
	put!(systemstate.radar_scan_queue, msg)
	# check if the queue still has space
	@show length(systemstate.radar_scan_queue.data)
	if length(systemstate.radar_scan_queue.data) < systemstate.radar_scan_queue.sz_max
	# nothing more to do
	return nothing
	end
	# type instability
	# Full sweep, lets empty the queue and add a variable
	queueScans = Vector{Any}(undef, systemstate.radar_scan_queue.sz_max)
	for i in 1:length(systemstate.radar_scan_queue.data)
	# something minimal, will do util for transforming PointCloud2 next
	println(i)
	md = take!(systemstate.radar_scan_queue)
	# @info typeof(md) fieldnames(typeof(md))
	pc2 = Caesar._PCL.PCLPointCloud2(md)
	pc_ = Caesar._PCL.PointCloud(pc2)
	# pc = Caesar._PCL.PointCloud(; height=md.height, width=md.width)
	queueScans[i] = (pc2,pc_)
	# queueScans[i] = pc_
	end
	# add a new variable to the graph
	timestamp = Float64(msg.header.stamp.secs) + Float64(msg.header.stamp.nsecs)/1.0e9
	systemstate.curtimestamp = timestamp
	systemstate.cur_variable = addVariable!(fg, Symbol("x$(systemstate.var_index)"), Pose2, timestamp = unix2datetime(timestamp))
	systemstate.var_index += 1
	io = IOBuffer()
	serialize(io, queueScans)
	# @show datablob = pc # queueScans
	# and add a data blob of all the scans
	# Make a data entry in the graph
	addData!( fg, :radar, systemstate.cur_variable.label, :RADARPC,
	take!(io), # get base64 binary
	# Vector{UInt8}(JSON2.write(datablob)),
	mimeType="/application/octet-stream/bson;dataformat=Vector{Caesar._PCL.PCLPointCloud2}",
	description="queueScans = Serialize.deserialize(PipeBuffer(readBytes))")
	#
	end