Optimise beam cube

[sjperkins]

• Tests added / passed

  $ py.test -v -s africanus

  If the pre-commit tests fail, install and
  run the pre-commit hooks in your development
  virtuale environment:

  $ pip install pre-commit
  $ pre-commit install
  $ pre-commit run -a

• Fully documented, including HISTORY.rst for all changes
  and one of the docs/*-api.rst files for new API

  To build the docs locally:

  pip install -r requirements.readthedocs.txt
  cd docs
  READTHEDOCS=True make html