Pyheadtail

**New features:**
- Migration to Python 3
- Documentation and small fixes in synchrotron module
- Sphinx auto-generated documentation available at http://pycomplete.github.io/PyHEADTAIL/

**This version of PyHEADTAIL is incompatible with Python 2!**


v1.13.5_python3
**Python 3 version of [PyHEADTAIL v1.13.5](https://github.com/PyCOMPLETE/PyHEADTAIL/releases/tag/v1.13.5).**

**This version of PyHEADTAIL is incompatible with Python 2.**

Hotfix:
- fixing issue https://github.com/PyCOMPLETE/PyHEADTAIL/issues/102 with NaN emittances when the beam has a zero-filled array in `dp`. Addresses both GPU and CPU implementations, as well as bunch emittance and slice emittance functions.

On the way to more and more python3 support:

cython requires `compiler_directives={'language_level' : "3"}` for python3 support -- in order to remain compatible with python2, we include


cythonize(extensions, compiler_directives={'language_level' : sys.version_info[0]})


into the `setup.py` (thanks to [stackoverflow](https://stackoverflow.com/questions/34603628/how-to-specify-python-3-source-in-cythons-setup-py)).

**Changes**:
- generate transversely cut distributions (e.g. a Gaussian distribution cut at 3.4 RMS amplitudes): supported in `generators.generate_Gaussian6DTwiss` via keywords `limit_n_rms_[x,y,z]` in all 3 planes.
--> The concept is demonstrated in the [PyHEADTAIL-playground repo (GeneratorTest)](https://nbviewer.jupyter.org/github/PyCOMPLETE/PyHEADTAIL-playground/blob/master/howto_notebooks/GeneratorTest.ipynb).

Changes:
- aperture bugfix when running on the GPU with slicemonitors (thanks Eirini!)

Few corrections (adapting license and adding pull_request_template) and adding Gianni's python 2-to-3 script.