Turn InferenceObjectsNetCDF into an extension

[sethaxen]

If we move from_netcdf and to_netcdf into InferenceObjects with a fallback that instructs the user to load NCDatasets to use these functions, then we can place the implementations in an extension. There seem to be some trade-offs to this approach, and it's not clear either way whether it should be preferred to our current subpackage approach.

[sethaxen]

An even better approach would be to just overload functions in the two files. e.g. for loading an InferenceData from a NetCDF file, we could just define the overload convert_to_inference_data(::NCDataset). The problem is writing to a NetCDF file. The NCDatasets API only provides functionality for iteratively writing data to a NCDataset, not a way to write an object to one.

If #16 is implemented, then a better solution would be to support an NCDataset being a store for an InferenceData.

By comparison, NetCDF.jl has ncwrite and ncread that we could perhaps overload.