Clearly support multiple versions of WZDx to allow consumers to produce/consume feeds of different versions

[j-d-b]

Currently, the version of this .NET library corresponds to the version of the WZDx specification that the models apply to.

This is limiting in two ways:

• The library can't properly follow SemVer because only the patch can be changed if the WZDx specification doesn't change.
• Clients can work with two different feed versions using the same codebase.

One way to resolve the above issues is to version the models using a namespace, e.g. IBI.WZDx.Models.v4 which would have all the v4.X models, however this would result in a lot of duplication because the changes between versions have been historically minimal.

Another approach is to use preprocessor directives to build an assembly and NuGet package for each version.