🧹 NeuralNetwork cleanup pt. 2 -- data loading

[lindapaiste]

Extends some others PRs which should be merged first: #1407 #1388

This is primarily a cleanup of the internal workings of the NeuralNetworkData class. Step 2 of many.

• Move file loading functions which are not NN-specific to utils/io.
• Move the logic for finding the data array into a new file loadData. This might be temporary. I would have put it in the nnUtils but that is a class for no reason.
• Prefer accessing instance properties like this.data.raw rather than passing them around as arguments.
• Modified a bunch of internal NeuralNetworkData methods such that they just set/update the class properties and don't return anything.

Possibly breaking changes:

• MIME type for JSON files changed from text/plain to application/json.
• nn.normalizeData no longer accepts an argument or returns a value.