Rascal json encoder

[agoscinski]

Implements a rascal json encoder from ase frames to a json readable by rascal (more important for the c++ side than python). Solves issue #363

TODO:

• add doc in the doc page how to use this
• polish python doc of json_dumps_frame

Tasks before review:

• [ ] Add tests, examples, and complete documentation of any added functionality
  • [ ] Make sure the autogenerated documentation appears in Sphinx and is formatted correctly (ask @max-veit if you need help with this task).
  • [ ] Write additional documentation in the Sphinx (not docstrings!) to explain the feature and its usage in plain English
  • [ ] Make sure the examples run (!) and produce the expected output
  • [ ] For bugfix pull requests, list here which tests have been added or re-enabled to verify the fix and catch future regressions as well as similar bugs elsewhere in the code.
• [ ] Run make lint on the project, ensure it passes
  • [ ] Run make pretty-cpp and make pretty-python, check that the auto-formatting is sensible
  • [ ] Review variable names, make sure they're descriptive (ask a friend)
  • [ ] Review all TODOs, convert to issues wherever possible
  • [ ] Make sure draft, in-progress, and commented-out code is moved to its own branch
• [ ] If committing any code in Jupyter/IPython notebooks, install and run nbstripout
• [ ] Merge with master and resolve any conflicts
• [ ] (anything else that's still in progress)

Remaining tasks, out of scope of this pull request: (optional)

• [ ] Make sure to add these points to the issues or to the meeting agenda so they don't get forgotten!