Re-consider handling of dynamic URL retrieval code

[jeanpaulrsoucy]

Currently for datasets with a dynamic URL, R code is retrieved from url_fun_r in datasets.json and executed using eval(parse(text = "")). Executing arbitrary code stored in another package's repository is probably not the best way to handle this, since the code is hidden from someone directly inspecting this package's source.

Since there are relatively few instances of dynamic URLs, this code should probably be (redundantly) stored in a CSV within the package itself, or otherwise embedded directly within the function. This would improve the transparency of this package.