Notes:
SPKitSum defines an addInput() function for connecting additional inputs and overrides getSample().
Calls SPKitProcessor::setInput().
getSample() requests a sample from each of its inputs and stores the summed result in outputSample.
The function returns 0 when all its inputs run out of samples, 1 otherwise.
Kai Lassfolk / University of Helsinki / kpl@elisir.helsinki.fi