Multichannel binaural rendering: Default LFE gain not applied

For binaural rendering of multi-channel signals, the default LFE gain (bin_lfe_gain) is not set to its default value if ommitted in the yaml-Configuration. The default value is given as BINAURAL_LFE_GAIN = 10 ** (5.5 / 20) in audiotools/constants.py. If bin_lfe_gain is not explicitly set, a gain-factor of 0.0f (linear factor, not dB!) is applied, i.e. the LFE removed from the binaural output.