class_docstr: rgain Solution. Found on this repo https://pypi.python.org/pypi/rgain
  rgain relies on GStream tools for the analysis of the replay gain
  According to this pagem the reference level should be set to 83 for
  the case of 'no preamplification' (as it is implemented in Essentia).
  https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-good/html/gst-plugins-good-plugins-rganalysis.html#GstRgAnalysis--reference-level
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code: def compute(self, *args):
      key = args[2]
      command = ["replaygain", "-f", "-d", "-r 83", args[0].routes[key]]
      output = subprocess.check_output(
          command, stderr=subprocess.STDOUT, universal_newlines=True
      )
      if output.endswith("Nothing to do."):
          y = float(output.split("\n")[1].split(" ")[-2])
      else:
          return esarr(np.nan)
  
      return esarr(y)
  
file_path: essentia/test/src/QA/replaygain/test_replaygain.py
imports: import sys
  import subprocess
  import essentia.standard
  from qa_test import *
  from qa_testvalues import QaTestValues
label_desc: Use image node_id 1 for calling the RGain obj's underlying member method code with example usage: obj.compute() and returns: esarr, esarr