Store npz file

### 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.