See if a Datapackage object is non mutable or not

[nikolaj-funartech]

For the moment, Datapackage objects are supposed to be non mutable. In that case, we should force the finalization of such object and test if the object has been finalized before any operation can be applied to it.

If a Datapackage object is mutable, then introduce such mechanism.

[nikolaj-funartech]

The main issue is that we use Datapackage objects and threat them by default as mutable. If we want to introduce the notion of immutability, we also must introduce ways to create them as immutable. This is a big change in the code (but not for the users of the libraries). Is it worth it? @cmutel : What do you think?