(+ 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)

- Extend calculations by abstracting over `f64` and  `tch::Tensors`.
- The "free parameters" distinction vanishes
  in favour of gradient-tracking declaration.
- Have the calculation leaves step back from `Parameters` to `Scores`,
  ranging freely over the real line.
- Possibly move calculations to CUDA.