POC spinning off a process to compute parallactic angles

[sjperkins]

• Tests added / passed

  $ py.test -v -s africanus

  If the pep8 tests fail, the quickest way to correct
  this is to run autopep8 and then flake8 and
  pycodestyle to fix the remaining issues.

  $ pip install -U autopep8 flake8 pycodestyle
  $ autopep8 -r -i africanus
  $ flake8 africanus
  $ pycodestyle africanus
  

• 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
  

[sjperkins]

This is not a complete solution, but exists to show that it's possible to spawn process for parangle computation and thereby avoid the GIL

[sjperkins]

My initial thoughts on a more complete solution would be a ProcessPoolExecutor associated with a RimeFactory, which would be available for use by RIME terms..