[ENH] merge `skchange` into `sktime`

skchange seems abandoned by its maintainers now, but has a number of nice detection algorithms based on sktime. https://github.com/NorskRegnesentral/skchange

FYI @tveten, @johannvk, is this accurate?

I would suggest to merge skchange completely into sktime to ensure that it remains maintained.

For this, the contents of the skchange directory can be moved into the detection directory entirely.

The one major issue are likely the tests, which in skchange are somewhat custom; in sktime, TestAll classes either replace the tests, or may have to be added. This should be investigated.

If it is possible, it would be nice to maintain attribution to @Tveten and @johannvk in the PR, but I am not sure if this is possible beyond just adding them in authors tags and attributes.