Added the calculation of Time Constant by simplelung model

First implementation of APRV() ventilator model.

Which is, according to *Chatgurn's taxonomy*, a pressure controled,
intermitent mandatory ventilation (PC-IMV) mode of mechanical
ventilation.

	modifié :         src/simvent-ventilators.js
	modifié :         test/obsplots.html

A little more cleanup

- Code re-writen using ESMAS2015 "class x extends y" syntax;

- Lung and ventilator models now based on top level models to wich they add specific properties;
- Parameters uniformised between models (as mutch as possible);

first version of the speed comparison benchmark

fixed nan problem in sv.PVCurve

- SygLung model added

- PVCurve ventilator model added

VDR default parameters changed