viz/generate_viz_data.py crashes when using find_exclusive_sets.py/find_sets.py with default settings

[matthewreyna]

The find_exclusive_sets.py and find_sets.py scripts save their results in TSV format by default and JSON format with an optional flag, but the viz/generate_viz_data.py script loads its results in JSON format. This difference causes the viz/generate_viz_data.py script to crash when using output from the find_exclusive_sets.py and find_sets.py scripts with their default settings:

ValueError: No JSON object could be decoded

The JSON file includes a few extra pieces of metadata, such as the minimum mutation frequency and the test name, that can be either added to the TSV file or omitted from the visualization.