Audiotaper

This implements a audio taper potentiometer like found in hardware mixer. 
IMHO the new interface and implementation is much more self explaining:

double minDB, 
double maxDB,
double neutralParameter
 
I have made all log pots to this new one, but set the range to the values it was before.
So this is just a refactoring task for now and there should be no notable change from the user point of view. 

In the future this change allows to implement "long" sliders to -70 dB or add a boost region above 1 to a volume slider.   



this type of documentation should be moved to the header file, putting each definition right next to the variable it's referring to.  That way users of the API only need to read the header file