Write geometry optimization trajectory to .xyz file

What has been done:

• Output file eT.trajectory.xyz is now opened as part of the hf and cc energy functions.
• When initialized and when a new geometry is set, this xyz file gets the new geometry appended, so it can be opened in visualization software like avogadro or vmd to view the geometries.

ToDo-list

The boxes below need to be checked before merge review will start

• Testing
  • All tests pass -rcheck flag enabled in setup.py
  • All tests pass -fbatch flag enabled in setup.py
• Versioning
  • Changelog.md is up to date
  • If merge into master: The patch number in parameters.F90 and changelog.md is up to date
• Website (Needs to be done before merge)
  • Updates (if any) to the website have been added
• Compliance with style guide
  • Do's and dont's
  • Documentation style