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:

[JostMigenda]

Some minor cleanup; otherwise this looks fine. Once we have a separate models directory and have tested that this works, I think we can remove the from_github function, which is just a duplicate of the default case, right?

[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.

[JostMigenda]

LGTM, thanks @Sheshuk!