Fix dataset curation scripts and release new datasets with 1700+ COVID-positive images

[NaomiTerhljan]

• Creates updated datasets and dataset curation scripts. The COVIDx V7A dataset and create_COVIDx.ipynb are for non-binary classification, and COVIDx V7B dataset and create_COVIDx_binary.ipynb are for binary classification. Both datasets contain over 1700 positive COVID-19 images.
• Fixes create_COVIDx.ipynb and create_COVIDx_binary.ipynb to relect Radiography database changes.
• Updates documentation accordingly