Update setup.py and README.md for use on PyPI

[bouweandela]

Update setup.py and README.md so we can publish the ESMValCore package on PyPI. See here for more information.

Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• This pull request has a descriptive title that can be used in a changelog
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Codacy code quality checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.

---

Needed for #84

[bouweandela]

The package can be tested by running:

conda env create -n test -f environment.yml
conda activate test
pip install fire fiona nc-time-axis numba prov[dot] yamale
pip install --index-url https://test.pypi.org/simple/ --no-deps esmvalcore

This will first install all dependencies using Conda or PyPI and then the esmvalcore package from the test version of PyPI: https://test.pypi.org/project/ESMValCore/

To build and upload the package I followed the instructions here and used the commands

python setup.py sdist bdist_wheel
python -m twine upload --repository testpypi dist/*