Clarify how different assets are supported and identified in multi-asset proofs

[dr-orlovsky]

The section on transfer proofs in the specification https://github.com/rgb-org/spec/blob/master/01-rgb.md#transfer-proofs implies that multiple assets can be transferred in a single proof:

Every RGB on-chain transaction will have a corresponding "proof", where the payer encrypts, using the payee’s dark-tag, the following information in a structured way: <...> a list of triplets made with:

• color of the token being transacted
• amount being transacted
• either the hash of an UTXO in the form (TX_hash, index) to send an UTXO-Based transaction or an index which will bind those tokens to the corresponding output of the transaction spending the colored UTXO.

We need more clarification on how the "color of the token** is specified

Related issues:

• 72

[dr-orlovsky]

Obsolete because of #85