General improvements of the repo

[Kostis-S-Z]

• Re-implement SDK functions following functional programming, instead of OOP to remove necessity of setting up a client
• Create registry to handle dataset-specific loading functions
• Single source of truth for environment variables
• Remove duplicate code by implementing a generic api_request() function to handle GET/POST requests
• Add proper docs

[Kostis-S-Z]

Closed with #43