[WIP] Add docs and tests

[espottesmith]

Description

Thanks to @punnaamorn's work, we now have the beginnings of a test suite. This PR will attempt to expand this, aiming for near-perfect test coverage. A goal of this PR is also to improve documentation. This is an important step towards an initial v0.1 release.

Change features

• Completed random sampling started in [WIP] Basic Data Transforms and Splits #4 (and, thus, transforms.jl) and added associated docstrings.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have run tests locally, and all tests passed
• My changes generate no (new) warnings