nn.test.js added, removal of duplicated code in nn.js

[GhostRabbit]

Wanted to remove code duplication in nn.js predict and train functions and as nn.js was not under any testsing I had to add at least some coverage test of the code I wanted to refactor.

nn.test.js Have been created with some very basic tests

nn.js Have been refactored. (I use VS Code auto-format on save-option so many minor code format changes have slipped throuhg here)

package.json Added 'npm run test-watch' command for faster local devlopment