First packaged release

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

• Re-structuring the repo.
• Add a proper documentation, https://pyfftlog.readthedocs.io, convert
  plain-text math into LaTeX, and add a reference section.
• Add example notebook as sphinx-gallery to docs.
• Add tests and CI on Travis, https://travis-ci.org/github/prisae/pyfftlog.
• Link to Zenodo, https://zenodo.org/record/3830366.
• PEP8 checking and coveralls (https://coveralls.io/github/prisae/pyfftlog).
• Add the relevant badges to README.