Defect transformations, optical properties and misc improvements.

1. Defect transformations (Bharat).
2. Support for optical properties (Geoffroy Hautier and David Waroquiers).
3. Improved support for some VASP output files (XDATCAR and OSZICAR).
4. Refactored compatibilities now uses YAML for ease of reading.

Bug fix release

1. Bug fix for get_xrd_plot.
2. Speed up XRD calculator by allowing specification of two theta ranges.
3. Minor improvements to Gulp caller.

Bug fix for XRD calculator and misc improvements.

1. Bug fix for unequal coefficients sizes in XRD.
2. Support for Ag radiation in XRD calculator.
3. Improved Procar class for extraction of information. (Germain Salvato Vallverdu)
4. Bug fix for extraction of GGA data from Materials API.

Improvements to XRDCalculator

1. XRDCalculator now supports disordered structures.
2. Minor speed ups and improvements.

Beta XRD calculator and dielectric constants

1. Initial beta version of XRD pattern calculator.
2. Pymatgen now uses spglib 1.6.0.
3. Update to Vasprun to compute static deilectric constants with DFPT in VASP.
   (Geoffroy Hautier)

Bug fix release

1. Quick bug-fix release that provides a better solution to Structure handling
   of properties instead of sanitizing MPRester structures.

Minor bug fixes

1. Bug fix for linear assignment, which may sometimes affect Structure
   Matcher results.
2. Minor improvement to the way grand canonical PDs work.