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!