Add / check for `previousProof` option when adding a proof to a proof set

[dlongley]

When adding a proof to a proof set, a previousProof option should be checked. The previousProof property of a proof has a value of one or more IDs that identify other proofs that must exist on a document prior to generating the new proof, i.e., the new proof signs over existing proofs as "dependencies" forming a "proof chain".

The ProofChain interface (currently empty and unimplemented) should be removed and "proof chains" should be implemented by checking for this option and ensuring that the input document includes only the required previous proofs, i.e., a new proof property with a value that holds all of the previous proofs (and only those) should be added to the document that is to passed to the suite to create the new proof. The new proof should then be added to the original entire set of all proofs (which may have other proofs that are not part of its previousProof.