dropna-strata-ceiling

[jgallowa07]

Description

This PR:

1. fixes a bug with "--drop-nan" functionality
2. Adds "--strata-ceiling" to group strata above some threshold together during training.

Closes #(replace this parenthetical with issue number)

Tests

Please describe the tests added to verify correct behavior.

Checklist:

• [ ] The code uses informative and accurate variable and function names
• [ ] The functionality is factored out into functions and methods with logical interfaces
• [ ] Comments are up to date, document intent, and there are no commented-out code blocks
• [ ] Commenting and/or documentation is sufficient for others to be able to understand intent and implementation
• [ ] TODOs have been eliminated from the code
• [ ] The corresponding issue number (e.g. #278) has been searched for in the code to find relevant notes
• [ ] Documentation has been redeployed