Take this opportunity to:
- Leverage {Parameters,Scores}::map to ease code.
- Allow null probability input.

(+ remove optional final gradient search option)

(had to fix a few things within this crate, and my PR is still pending there)

An aggressive refactoring of `bin/aphid/main.rs`.

The idea is to get a better idea
what the desired formal output of aphid could be.

The likelihood formula used when comparing observed triplet
with a discordant scenario subtly differed from original implementation.

(mostly because `tch` failed to build after system `torch` upgrade)