#33: made Parameter.load() and NeuralNet.load() class methods

[shb84]

Test

• Updated unit tests to reflect changes to Parameters.load() method

Refactor

• Converted NeuralNet.load(...) to a classmethod such that reloaded = NeuralNet.load("save_params.json")
  • Previous pattern: reloaded = NeuralNet(layer_sizes=[1, 2, 3]).load("save_params.json")
• Converted Parameters.load(...) to a classmethod such that reloaded = Parameters.load("save_params.json")
  • Previous pattern: reloaded = Parameters(layer_sizes=[1, 2, 3]).load("save_params.json")

Closes #33