Add() should accept IEnumerable

[swharden]

Content originally posted by @shirok1 in #33:

---

An unnecessary type requirement in signature of sg.Add()

List<double>.AddRange accepts an IEnumerable<double> as the parameter. However, sg.Add() asks for an explicit double[], which means that I can't directly use the List<double> but have to do a .ToArray() first, which obvious will waste the memory as large as the audio itself.