adopt the phutil persistence class

[corybrunson]

{phutil} provides the "persistence" class, designed for general-purpose use. It should replace the legacy "PHom" class in {ripserr}. This means

1. adding {phutil} as a dependency
2. coercing the output to class "persistence"
3. deprecating (and later removing) methods and any other support for the "PHom" class

(Currently, {tdarec} is the only reverse dependency, and it can already handle the "persistence" class.)