make classy pip-installable

[ntessore]

This contains a number of changes to setup.py to make classy easily installable via pip.

• The setup.py file is now in the root directory. This is necessary because pip copies all files from the level of setup.py to a temporary location.
• The python setup calls make libclass.a internally.
• The OMPFLAG can be passed as an environment variable, or as an option to python setup.py build:

  $ # to install without OpenMP support:
  $ OMPFLAG= python setup.py install
  $ # can also be passed to build directly:
  $ python setup.py build --ompflag=

In particular, these changes will make it possible to pip-install classy straight from the command line:

$ pip install git+https://github.com/lesgourg/class_public.git#egg=classy
$ # alternatively without OpenMP, e.g. on macOS with default clang:
$ OMPFLAG= pip install git+https://github.com/lesgourg/class_public.git#egg=classy

You can already test this using my fork:

$ pip install git+https://github.com/ntessore/class_public.git#egg=classy

[ntessore]

There were a couple of issues with this PR. In the end it was easier to build a setup.py outside of the CLASS repository. The pip-installable package that we use as a requirement when testing our code can be found here.