Block.from_signed_raw() usage is not obvious

Parsing a block is not obvious. You have to append to the raw part, the signature and a \n to make it work:

block = await bma_client(bma.blockchain.block, block_number)
block = Block.from_signed_raw(block["raw"] + block["signature"] + "\n")

I thought, from_signed_raw(block["raw"]) was broken, but I found it used this way in Sakia.

We should address this. Will it be fixed with GVA? Do we want to implement a from_json() in the meantime?