API: Split data schema and internal data schema into two separate structures

Create a separate data structure for parsed and processed API data (as described in the [ARCHITECTURE.md](ARCHITECTURE.md] file. Currently, we mix processed and unprocessed data in one class and that has significant issues.

@amrazek Will you look at it or should I?

The main problem here is designing nice internal API so that we don't have to write a lot boring code copying stuff from one structure to the other. Maybe we could have fun with custom decorators? 🤔