Use great_expectations to validate breast cancer data

[ttimbers]

@RohanAlexander shared this example of code using great_expectations to validate data. And here are the great_expectations docs: https://docs.greatexpectations.io/docs/core/introduction/try_gx/

What should we validate about the breast cancer data? Here are some ideas:

• data schema (column names and types)
• value ranges (we may have to extract this from the data themselves, or the original breast cancer paper since we are not domain experts ourselves)
• how we expect missing values to be recorded
• what percentage of missing values is OK