Audit the DTChecker implementation, simplify where possible

[ev-br]

The implementation of DTChecker has several "this needs to be done exactly like this" comments. On the contrary, some of these may be possible to simplify/streamline. However, this code has grown to account for actual usage in existing codebases, so this maintenance has a prerequisite of correctly testing at least numpy and scipy documentation.

[ev-br]

One smelly thing is https://github.com/ev-br/scpdt/blob/main/scpdt/impl.py#L317 : would be better to not create ragged arrays internally at all.

[ev-br]

Probably best to wait out until NumPy 2.0; then make sure it all works on both 1.x and 2.0, and only then go trim the fat.