Xrayutilities

* harmonize dependency versions between README and setup.py
* remove PDF documentation and other generatable content from the GIT
repository
* new azure pipeline for documentation generation
* new azure pipeline for wheel generation (issue 111)

* allow string decoding to fail in tif headers
* ignore nan values in maplog
* add Azure pipeline for continuous testing and automatic builds
* performance optimizations in Crystal.environment
* Continous integration tests running on MacOS, Windows and Linux
* new flavor of material/Crystal definition from fractional atomic coordinates
* bugfix in derivative functions (xu.math.functions)
* effectiveDensity slicing for XRR simulations, thanks to MM
* new possibility to transform unit cells and find symmetries
* bugfix of numpy.set_printoptions use
* require Python 3.6 due to use of f-strings
* new PowderModel.plot method to quickly visualize a Powder pattern
* remove deprecated fit_xrr function
* SGLattice.get_allowed_hkl function
* make SGLattice.convert_to_P1 a normal method
* SGLattice improvements: add symmetry operations, equivalent_hkls,
reflection_conditions, and allowed_hkl functionality
* fix SGLattice.isequivalent function to work for non-centrosymmetric
materials
* implementation of the March-Dollase model for preferred orientation in
PowderDiffraction
* Mayavi based visualization of unit cells: Crystal.show_unitcell
-> new optional dependency on Mayavi
* fix new Pilatus TIFF files which seem to have \xff padding
* implement Gaussian beam shape for XRR modelling
* update documentation to recommend the use of pip for installation
* remove multGaussFit/Plot and set multPeakFit/Plot deprecated
-> use the much more powerful lmfit-package instead!
* rename dummy element to "Dummy" and set sensible default parameters

* use of tox to run unittests consistently in a virtual environment
* build element database during installation from sources
* new source layout which offers many advantages
(https://blog.ionelmc.ro/2014/05/25/python-packaging/#the-structure)
* remove Python2 code path, improve code readability
* fix use of the agg matplotlib backend with FitModel
* from this point on Github master will be broken on Python 2

* fix issue 84: edge case problemes in XRDML parser
* matplotlib compatibility improvements

* issue 87: fix matplotlib inline backend in FitModel.fit
* enable parsing of Panalytical XRDML version >2.0
* issue 79: fix parsing of Sardana generated spec files; thanks to DS
* issue 81: fix parsing materials with space group 47
* packaging cleanup: remove gitignore and other dev-files
* issue 77: complete multiprocessing cleanup upon PowderModel.close()
* issue 76: cif parser: fix bug introduced in version 1.5.2

* fix bug in PowderDiffraction where the last Bragg peak before tt_cutoff was
omitted, thanks to NG for reporting the issue
* new model for calculating signal of mosaic small crystals
* add arbitrary line cut function
* fix parsing of empty scans in spec files
* remove geth5_map function -> use geth5_scan instead
* support inf/nan values in spec files
* setup.py fixes for Debian packaging
* CIF export: make _atom_site_label unique
* change of SMaterial.density to a unit of kg/m3
* deprecate the fit_xrr function, which will be remove in the next major
version -> use the new FitModel
* new FitModel class for fitting of instances of LayerModel
* fix bug in PowderDiffraction used with enable_simulation=False