Adding default option of downloading files from given URL, instead of fixed github one

[Sheshuk]

Following my suggestion on making a separate repository for the data files, I would like to add the capability of downloading the files from any given url.

URLs can be formatted with any parameters, so I implemented the same github downloading in a new format for illustration:

[Sheshuk]

I removed the from_github, cleaned up the docstrings, and also added a default parameter snewpy_version which can be put to the URL template. So it will always point to the current version.