kklmn/xrt: Release 1.6.0

- Enable calculations of elastically deformed crystals on GPUs. Based on PyTTE code. Used both in ray tracing and
  xrtBentXtal GUI -- a GUI for comparative GPU-based bent crystal calculations.

- Add predefined material classes in three categories: crystals, compounds and elemental. Several crystal classes also include
  elastic constants needed to calculate bent crystal reflectivity.

- All OpenCL tasks can be run on a remote GPU server.

- Extend the examples of 1D- and 2D-bent crystal analyzers with elastically deformed crystal reflectivity.

- Add docs "sampling-strategies" on sampling strategies of syncrotron sources. Add an example
  "Undulator radiation through rectangular aperture" that illustrates various sampling methods.

- Add user classes of optical elements to xrtQook.

- Add an example "Orbital Angular Momentum of helical undulator radiation".

- Use python properties in OEs, screens, apertures etc. to transparently set object attributes.

- Bug fixes.