Prepare for ChunkSplitters 3.0

[carstenbauer]

OhMyThreads API Changes:

• split can now be a Split or Symbol.
• we now re-export Split, Consecutive, RoundRobin
• we now re-export chunk, chunk_indices
• when users provide chunks or index_chunks don't throw a warning about the fact that we auto turn off our internal chunking. instead throw a warning if they set incompatible chunking related kwargs.

TODOs:

• [x] update "non-exported" section of the docs
• [x] throw warning if user provides chunks or index_chunks and also sets @set chunking=true or other chunking options like @set ntasks=5 etc.

[carstenbauer]

If one uses the linked PR of ChunkSplitters, all tests are passing locally.

[carstenbauer]

@MasonProtter would be great if you could give this a quick look. Would like to merge and release this in the next couple of days.

[carstenbauer]

Apart from the const vs using discussion above, this seems to be good to go.

(What one could do is walk over the docs and see if any of the examples could be simplified by using chunks instead of index_chunks. But that's neither important nor does it have to be done in this PR.)

[carstenbauer]

I'll merge this. We don't have to register the new version right away to maybe make other breaking changes (e.g. related to #124).