Simplify Result instantiation

[diego-plan9]

What is the expected enhancement?

From #1023 (please check for longer discussion), the process for finally instantiating a qiskit.Result would benefit from being more straightforward, specially as it is a class that is of relevance to backend implementators (including AER and third-party) and for for end users. That streamlining would imply relaxing a bit the idea that all schema-defined instances are "light containers", which in this case might be justified - specially depending on the outcome of #830.

Proposed checklist:

• unify the Results object, making use of qiskit.validation classes
• append the Result API methods
• update local simulators Local simulators should generate results according to the result schema #755

Related issues:

• Review qiskit.Result API #830: defining the Results API
• Add auto-validated objects machinery #1249: sets the basis

[diego-plan9]

Closing, as the remainder work is tracked at #755 .