Improve Load Parameters

[shb84]

Issue

Presently, in order to reload pre-trained parameters, the object must first be initialized:

import jenn
reloaded_model = jenn.NeuralNet(layers_sizes=[1, 2, 3]).load("parameters.json") 

This is clunky because passing layer_sizes should not be needed, given that it can be inferred from parameters.json.

Need

Also support the following pattern: reloaded_model = jenn.NeuralNet.load("parameters.json")