763 scgpt split data

[jakubmajercik]

Changelog

Data splitting helper functions added under utils. Possible to split data 2way or 3way.

Issue ticket number and link

Closes #763

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• [x] I have performed a self-review of my code

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  • [ ] Breaking changes
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• [ ] Proposed changes are described in the CHANGELOG.md

• [ ] CI tests succeed!