Release 2.2.1

• Adds the ability to perform QPA from a powder pattern when the crystalline phases are known, with an example notebook from the 1999 round Robin
• Solves an issue where updating the Crystal widget would append a js output for each update
• A few changes to setup.py to specify optional modules (but setup.py is not used anyway so this is mostly for documentation)
• Add python 3.8 as supported (should work in 3.9 as well but needs testing)
• There was a change in the previous version where the str() of a molecule used to return the XML output, but this has changed. A RefinableObj.xml() convenience function has been added
• Added a PbSO4 notebook example with joint X+N optimisation
• Solves an issue where using XMLInput() on a Crystal could lead to a crash if the structure was not empty and the scattering powers where added manually (not by importing a Molecule). DeleteRefParInDestructor is now correctly taken into account there, also with a hook to re-use existing scattering power when possible to avoid (minimise) memory leaks.
• Solves an issue with the import of urllib.request.urlopen
• When adding an Atom or a Molecule to a Crystal, an exception will now be raised if the used ScatteringPower are not already added to the Crystal (this could lead to a crash). Check this is OK for Diffpy-CMI
• Update doc:
  • add references specific to the recent changes (powder pattern, crystal 3D view, global optimisation,...) including me a contact.
  • update some info about installing from source
  • replace m2r by m2r2 for sphinx as m2r seems abandoned
  • added the example notebooks in the sphinx documentation (requires nbsphinx and nbsphinx-link). The javascript 3D crystal views are even preserved.