Widgets for multi-valued parameters

[DavidBerger98]

Enable/implement a dynamic widget for multi-valued parameters (e.g. tuples) similar to what has been done in the Vera example (http://kitware.github.io/simput/app/). It would be neat to be able to define a type for each field separately.

See picture below:

In this example we'd have tuples of (string, float64).

I would imagine the model.yaml to look like this:

MultiValuedTuples:
  _label: Multi-valued tuples
  _help: >-
    You can also define multiple types as a tuple, like this.
  type: [string,float64]
  size: -1
  initial:
    - ["a",1.0]
  domains:
  - type: UI
    properties:
      sizeControl: true