Improve download_test_db.py

[goldsborough]

Improves the script to download datasets a bit.

1. Adds an argument parser:

 psag@psag-mbp:~/home/Glow python utils/download_test_db.py -h                                                                                                                                                                                                                                                           
usage: download_test_db.py [-h]
                           [-d {ptb,mnist,cifar10} [{ptb,mnist,cifar10} ...]]
                           [-a]

Download datasets for Glow

optional arguments:
  -h, --help            show this help message and exit
  -d {ptb,mnist,cifar10} [{ptb,mnist,cifar10} ...], --datasets {ptb,mnist,cifar10} [{ptb,mnist,cifar10} ...]
  -a, --all

This allows downloading individual datasets with -d <dataset1> [, <dataset2>] or all of them with -a/--all. Closes #211.

Also adds a progress bar and better error handling and descriptive output: