`AggregateSignatures::from_signatures` and `MultiSignature::from_signatures` should accept a single signature

[JamesHinshelwood]

Currently, the input is validated to be at least 2 in length:
https://github.com/hyperledger-labs/agora-blsful/blob/49827e21b98902e686dcb39aa2c64a1716988257/src/aggregate_signature.rs#L84

However, I don't see why we shouldn't be able to generate aggregate signatures consisting of a single signature (even if that is effectively the identity function). I think this also aligns with the spec which states:

Inputs:

• signature_1, ..., signature_n, octet strings output by
  either CoreSign or Aggregate.

Outputs:

• signature, an octet string encoding a aggregated signature
  that combines all inputs; or INVALID.

Precondition: n >= 1, otherwise return INVALID.

It is slightly confusing, but I think the correct interpretation is that when n = 1, inputs = signature_1, ..., signature_1 which represents a single signature as the input.