Fix LaserScan metadata

[kawk]

Some metadata in the LaserScan messages seems to be inconsistent with the msg/LaserScan specification or not set at all.

These changes try to improve the content of the fields scan_time, angle_min, angle_max, angle_increment and time_increment. A more detailed description of the changes is included in docs/metadata.md (which might be a bit too much about internals to be included as is?)

Note that especially the changes to the angle information might not match expectations of existing subscriber implementations, while scan_time and time_increment weren't usable before anyway - as a side effect this probably fixes #4

The changes additionally try to avoid deduction of the metadata from static driver configuration. Instead, as much as possible information is gathered from the received packet header.

[JnsHndr]

I think that the value layer_count read from the device (R2300) should be assigned here.
params_->scan_time_factor = parser_utils::to_long(resp["layer_count"]);

If a R2300-4-layer (device_family==5) is connected, then params_->scan_time_factor = num_layers is first set to 4 which comes from config/r2300_params.yaml (call stack: main() ---> pf_interface.init()/PFInterface::init() ---> PFInterface::handle_version()) and later overwritten with params_->scan_time_factor = 1 (call stack: main() ---> pf_interface.init()/PFInterface::init() ---> protocol_interface_->update_scanoutput_config() ---> PFSDP_2300::get_scan_parameters()).

[kawk]

The scan_time_factor depends on whether there are separate LaserScan topics for each layer (R2300 4-layer). The current driver emits separate topics for R2300 4-layer but not R2300 1-layer, although both report layer_count=4. It probably should become a user option to choose separate or unified output for R2300 1-layer

[kawk]

In 9b97265 the scan_time_factor setting is done only once near PacketReader instantiation, to avoid later overwrite.

The purpose of num_layers YAML parameter is unclear to me. Until now it just seems to be expected to replicate the actual number of layers in sensor (which can be read from sensor as layer_count parameter).