PyFAI v0.18.0

• Last release with Valentin as he finishes his contract soon
• Almost 800 commits, 60 PR since the last release: this is a huge
  release !
• Major rework on all GUIs, mainly pyFAI-calib2 and pyFAI-integrate.
• Possibility to integrate image stacks (i.e. from HDF5), ...
• Rework the method to specify the algorithm, pixel splitting and
  implementation
  with sensible fall-backs. Also available via the different GUIs
• 3D visualization of detectors and experimental setup, useful for non
  flat detectors.
• integrate1d_ng is available with histogramming without pixel
  splitting in
  Python, Cython and OpenCL. Now, propagates the variance properly !
• IO sub-packages with associated refactoring for ponifile, json,
• Improved management of OpenMP: simplify the code for histograms.
• Improved geometry description and tutorial for writing exchange
  with other software (ImageD11, thanks to Carsten Detlefs).
• More reliable simple ellipse fitting with tests and doc.
• Better POCL integration (debugged on cuda, x87, Power9, ...)
• Rely on silx mechanics for the build, test, download, GUI,
  opencl ...
• Many new tutorials, now available on binder-hub: new calibrants,
  Pilatus calibration, ...
• Fix many issues reported in third-party software (Dioptas, ...)
• Drop support of debian8, Python 2.7 and Python 3.4 on all
  platforms. It is likely most functionalities still work but without
  guaranty.