title: Flask-restplus: how to define a nested model with 'allOf' operation?
body: <p>Creating a python flask rest plus server application,
  I'm trying to create a model for input body (in POST operation) with '<strong>allOf</strong>' operator,
  which is equivalent to the following example, taken from swagger.yaml I've created with the swagger editor:</p>
  
  <pre><code>definitions:
    XXXOperation:
      description: something...
      properties:
        oper_type:
        type: string
          enum:
            - oper_a
            - oper_b
            - oper_c
        operation:
          allOf:
            - $ref: '#/definitions/OperA'
            - $ref: '#/definitions/OperB'
            - $ref: '#/definitions/OperC'
  </code></pre>
  
  <p>It should be something like (just in my crazy imagination):</p>
  
  <pre><code>xxx_oper_model = api.model('XXXOperation', {
      'oper_type': fields.String(required=True, enum['oper_a', 'oper_b', 'oper_c']),
      'operation': fields.Nested([OperA, OperB, OperC], type='anyof')
  })
  </code></pre>
  
  <p>when OperA, OperB, OperC are also defined as models.
  How can I do that?</p>
  
  <p>Actually, I prefer to use '<strong>oneOf</strong>', but as I understand it's not supported even in the swagger editor, so I try to use the '<strong>allOf</strong>' with not required fields.</p>
  
  <p><strong><em>Versions:</em></strong> flask restplus: 0.10.1, flask: 0.12.2, python: 3.6.2</p>
  
  <p>Thanks a lot</p>