Now possible to add new Modifier to a Sound, as an Observer.
New condition on notifyObservers in set methods.
The modifiers are no longer observers.

Thus it is possible to apply them all before notifying the observers
that the data changed.
Add function applyModifiers and use it in generateSignal.
Create the Parameter interface.
Parameter is now a class of generic type.
Parameter is an abstract class.
getValue and setValue are now abstract in Parameter class.
Sound now implements Observer interface.

This way it can get informed when a Modifier changes.
ParameterEditor is no longer abstract.
Creates Modifier abstract class to represent any effect or filter.
Parameter now relies on a validation function.
Creates Modifier abstract class to represent any effect or filter.
Create IntegerParameter and DoubleParameter.
isValid has been replaced by parseValue for conveniency.
Merge branch 'master' of https://github.com/remigourdon/sound-editors