Multi Sensors in reader mode

this MR allows to use several reader at the same time in VeloView.

Timestamps of all frames have to match (start around the same time and finish at the same time) For example, you can open a single pcap in 2 readers by specifying the port to read on each of them.

should be merged after lidarview-core!220 (merged)

For all review see VeloView-Velodyne!84 (merged)