Message payload should return object on simple messages

[javihern98]

Change message payload to retrieve the sole object if only one of one kind is found:

• If a DataSet is present and it is only one, return the Dataset object
• If any ItemScheme or Definition is present, return the sole object
• If more than one is present, but of the same kind, return a list
• If more than one is present from different kinds, return same type as content

[guillegrc]

Instead of modifying the read_sdmx function, I have added the methods get_organisationschemes, get_codlists, get_concepts, get_dataflows and get_datastructures to the class message. I have modified the payload to return the sole object if it is only one Dataset too , I have modified a bit the returns in read_sdmx and added the read_xml to return the type of data. It is already committed all of it.

[guillegrc]

Add function get with an specific key for DataStructures, Dataflows, OrganisationSchemes, Codelists and Concepts.

[javihern98]

Add function with get_dataset with key, if there is only one, check dataset.unique_id = key, if it is not equal raise ValueError exception.