Load pre-trained neural network models and save trained models

[beomyeol]

This issue is brought from cmssnu/dolphin#104.

Current DNN does not support loading and saving neural network model parameters. Since it usually takes several hours or even several days to train neural network models, it is hard to train the models every time. In order to classify new data, pre-trained models should be used. Thus, we should make DNN load and save the trained neural network parameters.

[yunseong]

In today's meeting, we discussed that this feature - store/load pre-trained models - is not restricted to DNN; Instead, we can expose a general checkpointing mechanism for fault tolerance, and make DNN use that mechanism to store/load models.