Implement bin-free NFW profile realization technique

Thanks to @manodeep for pointing me to [this research note](https://arxiv.org/abs/1805.09550). Halotools is now in a feature-freeze for the soon-to-be-released v0.7, but this should be a relatively short-term priority since it's clearly a better method for generating Monte Carlo realizations of NFW profiles.

@johannesulf might find this technique useful.