i-PI integration

[max-veit]

Add support for i-PI using a dedicated calculator, which is also usable as a "generic" MD interface for any other MD code. Works with the branch https://github.com/cosmo-epfl/i-pi/tree/feat/librascal

The new calculator is implemented in bindings/rascal/models/genericmd.py.

Still to be worked out:

• Confirm sign convention of virial Confirmed experimentally
• Make units handling more robust (fewer assumptions) moved to i-PI
• Does i-PI run non-periodic simulations? If so, how is this signalled to the force driver? No

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
• Merge with master and resolve any conflicts

Co-written with Lorenzo (@lgigli)