Major clean up in dowload dataset functions in files.py

[JoelKronander]

fixes #64
replaces old PR#60
This PR represents a quite extensive clean up of the download functions in files.py.
Using a new utility function called maybe_download_and_extract() the individually, and repetitive definitions of helper functions in the dowload_dataset_X functions has been removed.

Additional functionality on top of more readable code are:

• Added a progress bar for file downloads, (particular convenient for larger file downloads such as wmt)
• Adding a new custom dataset download function is now significantly easier by using maybe_download_and_extract()
• The default path for data download is now set to data/ to be consistent with structure in examples/

All modified download functions have been tested, and all tutorials in examples/ runs properly.