Physical modeling synthesis, more waveforms

• New: Flexible waveguide model
  • Allow any input buffer to serve as a waveguide exciter
  • Support positively or negatively reflective waveguides
  • Add a non-linear limiter function for more realistic waveguides and to avoid over-excitation
• New: Add more physically modeled default waveforms
• New: Add StageSpec::Signal / SignalKind::Noise
• Improvement: Add LfSourceUnit::Wavelength