* 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