0: Original from Gnuspeech.
	Class: VocalTractModel0.
	The delay in each section is 1 period.
	Double precision.

1: Original from Gnuspeech.
	Class: VocalTractModel0.
	The delay in each section is 1 period.
	Single precision.

2: Equivalent to model 0, but the source code is easier to understand.
	Class: VocalTractModel2.
	The delay in each section is 1 period.
	Double precision.

3: Based on model 2, but the delay in each section is 3 periods.
	Class: VocalTractModel2.
	Double precision.

4: Based on model 2, but with 30 main sections.
	Class: VocalTractModel4.
	The size of the regions are: 3, 2, 4, 6, 6, 4, 2, 3 (sections).
	The delay in each section is 1 period.
	Double precision.

5: Based on model 4, but uses flow equations and a pole-zero radiation
   impedance.
	Class: VocalTractModel5.
	The delay in each section is 1 period.
	Double precision.
