First packaged release

First packaged release on PyPi and conda-forge. This includes:

• Re-structuring the repo.
• Link to proper documentation, https://pyfftlog.readthedocs.io
  (which is for pyfftlog), which contains as well the example.
• Add tests and CI on Travis, https://travis-ci.org/github/prisae/pyfftlog.
• Link to Zenodo, https://zenodo.org/record/3830366.
• PEP8 checking.
• Add the relevant badges to README.