Refactoring and documentation

[reiniervlinschoten]

The package is currently quite cluttered.
Some functions need to retrieve data from several API endpoints (e.g. getStudyData).
Currently this is done in the function body.
However this way other functions need to implement these calls themselves.
The package should be refactored to create special functions, and maybe save returned objects, so all functions can use this information.
This would also make the easier to develop.

The same goes for documentation.
All functions should have a docstring and comments indicating what exactly is happening.
This way, we can also create an overview of which functions there are in the package.
And move functions that are for internal use only to private.

[reiniervlinschoten]

In #51 I have started working on adding tests, please see the PR for what needs to be done
In #52 I have started working on making the tests for getStudyDataPoints pass, please see the PR for what needs to be done.

After these 2 PRs, we need to make sure all tests changed and updated in #51 pass.
All tests and code changes should work toward making sure output of the functions matches: tests/test_files/files_output