Test package functionalities, document user experience issues

[pitkant]

Test all functions in the package. Pay attention to the following details:

• does function documentation give enough information and context? Should there be (more) examples? Is the return value documented? (see R Packages 2e Function documentation: https://r-pkgs.org/man.html)
• Are all lines of code tested? Are there some lines that should not be tested, for example if they are related to some very rare condition (nocov)?
• Should there be more vignettes / articles? What is missing?
• Compared to other data retrieval packages, are there some features / function parameters etc. that would be useful?
• Are there some performance bottlenecks? Does loading data into R consume lots of RAM, does downloading data take lots of time, would caching be useful...?

Take notes when going through the package functions. You can post them under this issue or keep them locally, we can go through them later