Create working script for downloading the MNIST dataset

[jkglasbrenner]

Is your feature request related to a problem? Please describe.

There are multiple copies of the MNIST download script across the project that are used to download the data and save it as a set of PNG files. Examples include:

• https://github.com/usnistgov/dioptra/blob/0cdd17cad7985e37ad34640661e3470a1d02514b/examples/tensorflow-mnist-classifier/data/download_data.py
• https://github.com/usnistgov/dioptra/blob/290e2e16091796ac73499ea6f025167fbb438c80/examples/tensorflow-mnist-pixel-threshold/data/download_data.py
• https://github.com/usnistgov/dioptra/blob/6b516af2e11f589e746d98d5e78bf4f862576873/tests/integration/download_data.py

These should be consolidated and the script should be made available in a single, easy-to-find folder.

Documentation for how to use this script to download the MNIST dataset for use in a Dioptra instance will be handled in #154.

Definition of Done

• [ ] A working CLI Python script that downloads the MNIST dataset and stores it as PNG images is available in a /scripts directory
• [ ] Previous versions of the download script in the examples folders are removed

[jkglasbrenner]

Consolidate the MNIST downloader with the Roadsigns dataset downloader and annotation patcher: https://github.com/usnistgov/dioptra/tree/dev/examples/tensorflow-roadsigns-yolo/data

[jkglasbrenner]

Closed by #173