Make it clear distinction Mutable or not, and clear error messages

[mikibonacci]

We should provide

• clear and exhaustive error messages when immutablity creates exceptions (i.e. we try to modify immutable structures)
• When the aiida-atomistic is imported into aiida-core, make it clear exceptions if the package is not found (stating that the user needs to install it and so on)
• .get_value() methods instead of to_mutable.
• For the viceversa, just put a class method in the StructureData which is from_mutable or something like that.
• in passing by, fix the get_defined_properties (see the DFT+U example in notebook 3, the method does not show the magmoms.)
• Update the documentation and the tests! mainly the get_value and from_mutable methods.

[mikibonacci]

Just to mention where I improve this. For example, I modified the __setattr__ of the StructureData:

def __setattr__(self, key, value):
        # Customizing the exception message when trying to mutate attributes
        if key in self.__fields__:
            raise ValueError("The AiiDA `StructureData` is immutable. You can create a mutable copy of it using its `get_value` method.")
        super().__setattr__(key, value)