Add overview over response structures

[pr130]

closes #23

This PR adds a (rather long) RMarkdown report that provides insights into the structure of various asset types for GET requests.

The report is structured in the following (main results / challenges commented here): specific use cases

• Downloading files (e.g. xls / csv exports from survey submissions or XLS forms) seems to be a bit tricky.
• permissions are easily accessible for all assets as a data frame
• survey submissions are conveniently serialized as a data frame
• most basic information is available via the assets endpoint but all of it is also in the url endpoint for each specific asset (see below)

comparison across asset types -> is there information which we only get from the "general" assets endpoint and which information is only available in the more detailed data and url endpoints for each asset?

• all asset types have the same information in the url endpoint
• assets exposes most meta information but not all of it
• the data endpoint is only relevant for the survey assets, it exposes the submissions
• all asset types expose a list/data frame representation of their XLSForm / structure in the content element of the url endpoint data

detailed look at the asset types

• please refer to the document

[dimitri-mar]

Great Work!!

[pr130]

Great Work!! I just wonder if the .xls files should be committed or not. :)

yeah maybe not necessary but i thought it might be interesting to keep them :) also, the download somehow did not work in the end (i don't remember what i changed) so it's not reproducible right now via the code :D