- Add file CHANGELOG.md file to keep track of changes
  - Update Netgen to latest version
  - Changes to allow starting the Netgen GUI from Python:
    + To start the GUI from Python, just put 'import netgen.gui'
	in your python script. Some options (load python file,
	start python shell) are not available in this mode.
	On Windows, it's necessary to run the tk event loop manually:
        > netgen.gui.win.tk.mainloop()

        We need to address this, possibly by calling the mainloop
        from another thread.

    + Some cleanup (pyenv is only needed in the pdeparser, so declare
      it in pdeparser.cpp instead of python_ngstd.hpp)