Config refactoring

[JohanMabille]

The main changes are the followings:

• use std::optional to store the CLI values. This allows to get rid of sentinel values to check if an option was defined..
• merged the template classes Configurable and Wrapper into ConfigurableImpl.
• renamed the WrapperBase class into ConfigurableImplBase. This class holds all the data members that do not depends on the template parameter.
• renamed the ConfigurableInterface class into Configurable. This is now the unique interface to use for manipulating configurable objects.
• Removed all the over-duplicated API. The only remaining methods are those implicitly depending on the template parameter but where we cannot deduce it (yaml parsing, json dumping, etc ...)
• Fixed const-correctness issues.
• Removed suboptimal copies of Configurable.

[JohanMabille]

@wolfv I think this is ready to go.