Add option to provide buffered regions to embedders with neighbours

[RaczeQ]

GeoVexEmbedder and ContextualCountEmbedder don't inform the user if the regions_gdf doesn't have any neighbours that are required for proper calculations. It will just fail down the line.

Potential resolutions:

• Display a warning if any regions don't have all their neighbours
• Raise an exception if zero regions have required neighbours in the dataset
• Add option to pass an additional list of regions used to calculate embeddings and pass buffered regions as regions_gdf