Fl-data-downloader

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* get_dataset_for_course assumed all run numbers were sequential which resulted in datasets not being downloaded when they weren't!

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* resolve issue affecting all downloads after the FutureLearn website changed

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* added 30 seconds delay after a connection error before retrying

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* added max_retries to allow downloads to be "retried" if they fail.

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* added countries and country-subdivisions datasets
* removed index from CSV files created by fl-data-dl

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* updated default cache directory
* doc updates