Parameter Refactoring : String Parameter

The target branch of this MR is param_refactoring
This MR presents the new StringParameter class. It derives from the SingleParameter class. Here is its API :

public :
virtual void SetDefaultValue( std::string val)
virtual const std::string & GetDefaultValue() const
virtual void Reset() override
//Public coming from the SingleParameter class
virtual void SetValue( const std::string & val , int i = 0 ) override
virtual std::string GetValue( int i = -1 ) const override
//Public coming from the Parameter class
/*(function toggling boolean such as Mandatory, Active, UserValue, UserLevel 
or dealing with Role, Root, Key, Name and Description are omitted)*/
virtual void ClearValue()
virtual std::string GetValue( int i = -1 ) const = 0;
virtual void SetValue( const std::string & val , int i = 0 ) = 0;

If you have any ideas or comments on what we could add to the API of the parameter, feel free to express yourself!
As this kind of MR has not been done for the other classes (Parameter, SingleParameter, NumericalParameter) you can also comment on it here.

Associated MR !36 (closed)