Nnet rewrite (#105)
Nnet rewrite (#105)

* more rustic nnet create_weights fn

- use `windows` iterator
- use `rand::distribution::Range` as advised on `gen_range` method doc

* remove unecessary grad intermediary vector when computing gradients

* use  fn for BaseNeuralNet<BCECriterion>

* address @AtheMathmo comments

- revert back the `sum` iterator for a simple for loop
- revert back the `take` iterator instead of slice range
- set `gradients` with a default capacity of `weights.len()` (small speedup)

DarkDrek/rusty-machine,AtheMathmo/rusty-machine