Add initial test framework for utils functions

Added the initial test framework that will eventually encompass all of the functions in the package. Started with the primary utility functions since those are widely used and also simple enough to get used to writing tests. All current tests pass 😁

Fixes #57

added 12 commits

• df8d1a68 - 1 commit from branch rc:patch-v0.5.1
• 37f00456 - 1 earlier commit
• 739f5356 - add flake8 plugin for pytest linting
• a9f68667 - add pytest datafiles plugin
• 80e482bf - remove flake8 plugin
• dfff5f38 - add ruff linter and formatter
• 5b1c794a - add initial testing of utility functions
• 6f887736 - specify the names of the functions to import instead of *
• 70560988 - add test for size group assignment
• bda1b895 - add initial datetime tests
• 2c213b87 - finish tests for age grouping. combine days and weeks params into single...
• 81ff7e08 - apply ruff formatting

Compare with previous version

merged