improve example html. Add protocol field to proof output

[krlosMata]

• example
  • show browser console proving key loaded and witness loaded when proving_key.bin and witness.bin are loaded
  • disable calcProof button until files are loaded
• src/groth16.js
  • add protocol:groth field to object returned
  • needed when call snarkjs generatecall since it uses proof.json and protocol should be defined

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