Union semantics and variant enumeration order for tag bits

Currently, the spec says:

https://abs-tudelft.github.io/tydi/specification/logical.html#union-semantics

```
The "tag" field is used to represent which variant is being represented, by means of a zero-based index encoded as an unsigned binary number.
```

Do we want to allow custom tag enumeration of union variants?
If yes, we should add this to the spec.
If no, we should make explicit that the current text refers to the field index in the union declaration.
In the example, the tag enumeration is done by order of variant appearance in the union declaration, which seems most logical and straightforward, and it is similar to what most programming languages do as far as I am aware.