Introduced PacketRateCalculator.

[kaomoneus]

PacketRateCalculator allows to calculate rate of incoming packages.
E.g. when you want to extract FPS, or any other processing rate.

• As input it accepts any packets stream.
• As output it emits rate (floating point scalar),
  namely amount of packets per second based
  calculation of current packet and previous packet.
• For very first packet it emits empty packet.

Usage example:

node {
  calculator: "PacketRateCalculator"
  input_stream: "image"
  output_stream: "image_fps"
}

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