Add VectorDatabase method to purge records

[maciekbanas]

This one takes into account also changes from #83 and #84

[maciekbanas]

@kalimu if we pass all record IDs from the namespace to purge_records(), then the whole namespace is removed from the Pinecone. We can also add a method to remove simply whole namespace (I set up an issue already: #86) without need to pass the record IDs.