Allow users to override the EunomiaDatasets URL with an optional environment variable

[ablack3]

In the soon-to-be-released version of Eunomia users will be able to choose from multiple Eunomia datasets. These datasets are hosted in a separate github repo called EunomiaDatasets. It would be helpful if users could override this location and point to a fork of EunomiaDatasets for testing.

Currently the location is hard coded here:

Eunomia/R/EunomiaData.R

Line 59 in 06f0d3e

baseUrl <- "https://raw.githubusercontent.com/OHDSI/EunomiaDatasets/main/datasets"

Concrete use case:
I created a PR on EunomiaDatasets to fix an issue but I have no way to test that that my change works until after it is accepted. Ideally I'd like to be able to point to Eunomia to my personal fork and test that my change fixes the issue. I would propose using an environment variable for the location with the current location being used if the environment variable is unset.

What do you think @fdefalco?

[ablack3]

Thank you! This change should allow us to set up an S3 bucket with example datasets that are larger than what will fit in github.