The purpose of such refactoring is to allow for a "memoisation" approach
(i.e. avoid re-computing the same elements/metrics multiple times).

By redefining the API as a one-entry point function `evaluate`
(as opposed to calling each metric in turn), it makes it possible to
know in advance the metrics required by the user, and by keeping a
register (hard coded for now) of what unitary computation elements
each metric requires, and which metrics are nested in other metrics,
it is possible to avoid computation repetitions.