Pyheadtail

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.

Changes:

- license: 3-clause BSD license
- wakes: adding vertical parallel plates, renaming horizontal ones
- longitudinal: adding square-well potential (ABS beams)
- feedback: ideal transverse damper now also includes reactive part (see [PyHEADTAIL-playground](https://github.com/PyCOMPLETE/PyHEADTAIL-playground/))
- monitors: fixing CellMonitor also for GPU (now used in production)
- wakes: bug fix in resistive wall scaling
- generators: bug fix in kv2D
- context management: fixed and new functions
- generators: function to accept generated particles only within certain action (useful especially for linear generators)
- setup.py: speeding up cython (removing profiling flag)

Changes:
- hotfix: CUDA 9.1 compatibility
(thrust::gather has been moved to thrust/gather.h)
- include cellmonitor (cf. https://github.com/PyCOMPLETE/PyHEADTAIL/pull/75)