vocoder: add a psola-like vocoder

- modifiy Vocoder.h so that SPTK functions are protected instead of
  private and as such become reusable within child classes
- add a psola-like vocoder. For now it only generates a pulse-train
  in voiced parts and is equivalent to the basic vocoder, only it
  has an implicit delay of PSOLA_MAX_T0 samples.

actual psola-based methods could be implemented on top of this
vocoder: a bit more cleaning in the inheritance
psola: fail basic C++ inheritance ...