VolumePluginAudioUnit properties for busses and instance variable for buffer

VolumePluginAudioUnit.m

// Define parameter addresses.
const AudioUnitParameterID myParam1 = 0;

@interface VolumePluginAudioUnit ()

@property (nonatomic, readwrite) AUParameterTree *parameterTree;
@property AUAudioUnitBus *inputBus;
@property AUAudioUnitBus *outputBus;
@property AUAudioUnitBusArray *inputBusArray;
@property AUAudioUnitBusArray *outputBusArray;
@end

@implementation VolumePluginAudioUnit {
    Buffer _buffer;
}
@synthesize parameterTree = _parameterTree;