Serialize the Optimization Problem name

[vxfield]

Short descriptions

When you serialize an Optimization Problem object the name is not also serialized. This would be useful for customers who want to save a problem they've created locally and reuse it later without having to specify the problem name again.

Repro steps

1. Create a problem and serialize it:

problem = Problem(name="myProblem")
problem.terms = [
    Term(c=3, indices=[1, 0]),
    Term(c=5, indices=[2, 0]),
]
serialized_problem = problem.serialize()

Observed behavior

The value of serialized_problem does not contain the name of the problem ("myProblem" in this case):

{
  "cost_function": {
    "version": "1.0",
    "type": "ising",
    "terms": [
      {
        "c": 3,
        "ids": [
          1,
          0
        ]
      },
      {
        "c": 5,
        "ids": [
          2,
          0
        ]
      }
    ]
  }
}

That forces the user to specify the problem name again when deserializing the problem:

deserialized_problem = Problem.deserialize(problem_as_json=serialized_problem,
                                           name="myProblem")

Expected behavior

When serializing the problem, the name should also be serialized, like the example below:

{
  "metadata": {
    "name": "myProblem"
  },
  "cost_function": {
    "version": "1.0",
    "type": "ising",
    "terms": [
      {
        "c": 3,
        "ids": [
          1,
          0
        ]
      },
      {
        "c": 5,
        "ids": [
          2,
          0
        ]
      }
    ]
  }
}

And then, the user does not need to specify the problem name again when deserializing the problem:

deserialized_problem = Problem.deserialize(problem_as_json=serialized_problem)

Backward compatibility

Still, in case we are deserializing a problem that does not contain the name of the problem, the user should be able to pass a problem name again or, if not passed, we should have a default problem name set.

deserialized_problem = Problem.deserialize(problem_as_json=serialized_problem,
                                           name="myProblem")

[vxfield]

Resolved with #64