Add stratified sampling for NodeSplitter

[PantelisElinas]

Description

User Story

As a:
I want:
so that:

Done Checklist (Development)

• Assumptions of the user story met
• Produced code for required functionality
• Branch and Pull Request build on CI
• Branch and Pull Request pass unit tests on CI
• Branch and Pull Request pass integration tests on CI
• Version number reflects new status
• Peer Code Review Performed
• Code well commented
• Documentation on Google Docs
• Documentation in repo
• Team demo
• Mini-meetup talk
• Stakeholder sign-off

Done Checklist (Research)

• Code Review
• Documentation on Google Docs
• Documentation in repo
• Team talk
• Mini-meetup talk

Done Checklist (Bug)

• Bug fixed
• Branch and Pull Request build on CI
• Branch and Pull Request pass unit tests on CI
• Branch and Pull Request pass integration tests on CI
• Version number reflects new status
• Peer Code Review Performed
• Code well commented

[huonw]

NodeSplitter no longer exists (#885/#887), so moving this to Done.