Polish Qobj implementation

[delapuente]

What is the expected enhancement?

After the merge of #589 and #655, there are some "internal cleanup" tasks to be performed:

• reduce the amount of verbosity when creating subclasses (long term, it should be possible to recreate them from the .json specs themselves), ideally preserving facilities for static analysis and clarity for the user Streamline and make qobj classes more friendly #654
• update the tests so the QObjs built into them are not created from a dictionary Update the tests so the Qobjs built into them are not created from a dictionary #650
• revise the from_dict and as_dict methods, as they are currently mostly a convenience for the transition Revise the from_dict and as_dict methods, as they are currently mostly a convenience for the transition #652
• add more tests about the QObj class itself Add more tests about the QObj themselves #653