Support sandbox-based controls for license, grant, community

[ThierryO]

I'm creating a unit test for my upload_zenodo() function using https://sandbox.zenodo.org. However some methods of ZenodoRec require access to https://zenodo.org. I don't want to give my unit test functions access to https://zenodo.org.

This PR allows to pass a ZenodoManager object. The default behaviour is unchanged (use the default ZenodoManager pointing to https://zenodo.org).

[eblondel]

@ThierryO i've looked at this proposal. Thanks for spotting out this need to control licenses, grants and communities, this is really useful to improve the code. On this PR however I've opted for a different approach to make it more readable and easily usable, which consists in enabling a sandbox boolean argument in ZenodoManager, and propagate it through the methods setLicense, addGrand, addCommunity.

[ThierryO]

I still get an authentication error using ZenodoRecord$setLicense("GPL-3.0", sandbox = TRUE).

It looks like sandbox = TRUE does not update private$url.

zen4R/R/ZenodoManager.R

Line 89 in b78dca1

private$url = url