Initial Gain Coefficient tweaks

Initial Gain Coefficient tweaks
1)  Disambiguate 1.0 to GAIN_COEFF_UNITY, and 0.0 to GAIN_COEFF_ZERO
2)  Add GAIN_COEFF_SMALL which replaces SMALL_SIGNAL (-140dB)
3)  GAIN_COEFF_SMALL can used to avoid interpolating towards -inf on a db scale
4)  GAIN_COEFF_SMALL is used to detect very small (denormal?) gains and memset to zero