Rewrite `DenseMatrix` with constant alignment and less unsafe code

lightmotif-bench/dna.rs

lightmotif-io/src/jaspar/parse.rs

lightmotif-io/src/jaspar16/parse.rs

lightmotif-io/src/transfac/parse.rs

lightmotif-io/src/uniprobe/mod.rs

lightmotif-io/src/uniprobe/parse.rs

lightmotif-py/lightmotif/io.rs

lightmotif-py/lightmotif/lib.rs

lightmotif/benches/max.rs

lightmotif/benches/score.rs

lightmotif/benches/threshold.rs

lightmotif/src/dense.rs

lightmotif/src/num.rs

lightmotif/src/pli/mod.rs

lightmotif/src/pli/platform/generic.rs

lightmotif/src/pli/platform/neon.rs

lightmotif/src/pli/platform/sse2.rs

lightmotif/src/pwm.rs

lightmotif/src/scan.rs

lightmotif/src/scores.rs