OpenMP test branch

[ceriottm]

Let's open - at least as a playground - this can of worms. Is it possible with manageable effort to get openmp parallelization of the loop over centers that appear in all of the time-consuming step of the calculation?

Rationale and detailed description of the changes:

• added the minimal amount of infrastructure to Iterators to get parallel for loops to work
• added placeholders for the loops that could be parallelized
• at the moment, any actual parallelization breaks all the tests

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:

Parallelization across nodes, best left to the calling code's domain decomposition