WIP: Use pep 517 pyproject.toml to specify build dependencies

@mtr Did some quick work on this.

1. Specified a pyproject.toml to take advantage of pep 517 builds.
   • Solves chicken and egg build problem (pybind11 has to be installed before you can install your C++ extension)
   • https://www.python.org/dev/peps/pep-0517/
   • https://medium.com/@grassfedcode/pep-517-and-518-in-plain-english-47208ca8b7a6

The following should install correctly into a Python virtual environment.

$ python3 -m virtualenv -p python3 /tmp/ducc
$ /tmp/ducc/bin/pip install </path/to/ducc>
$ /tmp/ducc/bin/pip install -e </path/to/ducc>

Additionally, the following:

/path/to/ducc $ python setup.py sdist

should create a source distribution: something like dist/ducc-0.1.0.tar.gz, which is pip installable:

$ /tmp/ducc/bin/pip install /path/to/ducc/dist/ducc-0.1.0.tar.gz

The source distribution is what should be uploaded to PyPI