Validation of output tarballs

[AndreasAlbert]

With this PR, Submission.create_files now automatically calls the hepdata validator on the output file. An AssertionError is thrown if the tar ball is found to be invalid.

Closes #160

[AndreasAlbert]

I also added a new remove_old option to create_files. If set to True, the output directory is deleted before the new files are written. This ensures that no unrelated files accidentally end up in the tar ball, which the validator would identify as an error. Since many of our notebooks use the same output directory naming, consecutive execution of the notebooks -- as done in our tests -- can provoke this error if the folder is not removed between executions.

[clelange]

Lots of hidden gems in this PR ;-)

[clelange]

I forgot, shall we remove Python2 support with this PR? That would mean removing it from https://github.com/HEPData/hepdata_lib/blob/master/setup.py#L30

[AndreasAlbert]

Removed python2 from setup.py now

[clelange]

Thanks, and there goes Python2!