Make `Score` pipeline trait generic over the score type

lightmotif-bench/dna.rs

lightmotif/benches/score.rs

lightmotif/benches/threshold.rs

lightmotif/src/dense.rs

lightmotif/src/pli/dispatch.rs

lightmotif/src/pli/mod.rs

lightmotif/src/pli/platform/avx2.rs

lightmotif/src/pli/platform/generic.rs

lightmotif/src/pli/platform/sse2.rs

lightmotif/src/pwm.rs

lightmotif/src/scan.rs

lightmotif/tests/dna.rs