Add MLRWP - Chain Ladder to NN

https://www.youtube.com/watch?v=pBMQ3q0AFGo&feature=youtu.be

https://institute-and-faculty-of-actuaries.github.io/mlr-blog/post/research/chain_ladder_to_individual_mdn/

Start by reading the linked YouTube walkthrough and actuarial blog post, since the issue names no notebook, file, or test. Inspect the cookbook's existing notebook structure, then use those references to define the expected MLRWP Chain Ladder to NN result and verify the completed example against it.

索引モデルが issue の本文から書いたものです。