Change NIP license from "public domain" to CC0

[MMachado05]

The term "public domain" is one rarely standardized across jurisdictions, meaning that NIPs being defined as belonging to the public domain in one region/state is a different legal statement from being defined as such in a different region/state. The CreariveCommons 0 license aims to remedy this by explicitly defining what is meant by "public domain" so that these ambiguities do not lead to legal discrepancies across regions, thus universally maintaining the freedom to use the NIPs as any individual across the globe sees fit.