Store npz file

[jwblangley]

Summary

As per my own experience and Issue #77, I believe that the ability to save an npz file is a significant missing feature to this otherwise great tool! So I went and implemented it! The user can simply specify the --save-stats flag and the stats of the first path will be generated and saved in the location of the second path.

Changes

• Add the argument --save-stats to the argument parser
• Implement save_fid_stats using calcualte_fid_given_paths as a guide and making use of np.savez_compressed
• In __main__, if the --save-stats flag has been specified, run save_fid_stats and exit early. This design makes it clear that the --save-stats flag is the option and the original workflow is still the primary workflow.
• Update README.md to document the changes
  • My editor strips trailing whitespace. I believe that this is best practice so I have kept these changes in this PR.

Test Plan

I have manually tested the new workflow and the original and they behave exactly as expected. By logical inspection of the changes I've made, I believe no errors will have been introduced, however, I failed to run the included test suite due to an error that is present on master - prior to my changes. The error I am receiving is:

TypeError: compute_statistics_of_path() got an unexpected keyword argument 'num_workers'

Please advise how to fix this and I can rerun the test suite.

Code formatting

I have done my best to copy the style in which the existing code is written. I could not find a linter that is in use - if there is one, please make me aware of it.

[allglc]

Thanks for the implementation, it works well for me!

[mseitzer]

Thanks for the PR, looks good to me!