[DataComputed] Add support to compute a value with multiple origin

[titouanmathis]

As of now, we can only use the current group of Data... components to compute a value:

<input data-component="DataModel" type="text">
Length: <span data-component="DataComputed" data-option-compute="value.length"></span>

It might be useful to be able to use the value of multiple groups of Data... components, for example to add multiple values:

<input 
  data-component="DataModel"
  data-option-name="a"
  type="number">
+
<input
  data-component="DataModel"
  data-option-name="b"
  type="number">
=
<input 
  data-component="DataGroupComputed"
  data-option-names="a b"
  data-option-compute="a.value + b.value"
  type="number"
  readonly>

We could also have a similar DataGroupEffect.