fix: Fix CrossValidate to have tests for when data too small

[robertleeplummerjr]

Also upgrade to use newer es6 syntax for defining optional properties.
Upgrade examples to be more straightforward when using CrossValidate.
Fix values from being set to NaN when training with smaller data in CrossValidate.

Description

Fixes #185.

Motivation and Context

issue

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Author's Checklist:

• My code focuses on the main motivation and avoids scope creep.
• My code passes current tests and adds new tests where possible.
• My code is SOLID and DRY.
• I have updated the documentation as needed.

Reviewer's Checklist:

• I kept my comments to the author positive, specific, and productive.
• I tested the code and didn't find any new problems.
• I think the motivation is good for the project.
• I think the code works to satisfies the motivation.

[mubaidr]

Should be merged! Anything missing?

[robertleeplummerjr]

nope, just busy busy. Ty!