input/output gain read from json should be in dB

[falkTX]

right now the gain compensation uses a coefficient, but that not something commonly used for users. best to use dB as unit and do the conversion to a coefficient ourselves. basically...

            if (model_json["out_gain"].is_number()) {
                output_gain = DB_CO(model_json["out_gain"].get<float>());
            }
            else {
                output_gain = 1.0f;
            }