Add Global Names Verifier as a source and deprecate Global Names Resolver

[Rekyt]

Hi taxize maintainers,

taxize access Global Names services for taxonomic names resolution. Especially GlobalNames Index and GlobalNames Resolver. However, a more up-to-date version of the service (more efficient and faster) is now available with GlobalNames Verifier https://github.com/gnames/gnverifier. It would be nice if taxize could also provide access to it, as according to the developer, this is going to get all software updates from now on, while GlobalNames Resolver will only have its data updated.

As most of the API is similar to GlobalNames Resolver I suppose it shouldn't be that difficult to add a gnv_resolve() function to taxize.
However I know that the developer time is the limiting factor for the development of taxize, so I'm filling this issue to note this fact.
I may be able to submit a PR in the coming months at best.

Thanks!