Simulator bug fixes (proportion, repeat, yaml ingestion)

[snarayan21]

Description of changes:

Issues fixed:

• Proportion of None instead of a string 'None' is now handled correctly.
• Repeat of None instead of a string 'None' is now handled correctly.
• Tested using yamls multiple times, working correctly.
• Added warning for StreamingDataset subclass defaults, since we cannot depend on those subclasses directly.

Issue #, if available:

Merge Checklist:

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