as_diagram function

[corybrunson]

It might be valuable to have a coercion/conversion function as_diagram() that could take a persistence object and convert it to the format that {TDA} recognizes. This could serve to prevent problems with the deployment of {TDA} tools on persistence data, or just alleviate concerns.

Since we plan to deprecate the PHom class in {ripserr}, i see no point in implementing as_PHom.persistence(); and most persistence data sets aren't recoverable from hclust()—that is, they aren't in the image of as_persistence.hclust()—so there's no point writing as_hclust.persistence() either.

[corybrunson]

... although as_diagram.PHom() would be valuable! I'm doing this in the as-diagram branch.