[Python] Concepts rename pt2

[BethanyG]

• Changed the name of str-methods back to string-methods
• Renamed the stub files for "little sisters essay" to string_methods.py and string_methods_test.py
• Changed the stub files for "Currency Exchange" to numbers.py and numbers_test.py
• Updated the config.json files for "Little Sisters Essay" and "Currency Exchange"
• Edited the code examples in concepts/numbers/about.md
• Updated the exercises/README.md file with new "Currency Exchange" (numbers) exercise & links
• Updated the track config.json with UUIDs and new concept names
• Updated terms and added more links to python/reference/README.md

[BethanyG]

Added reference concept exercise table updates.