update to astropy template

[lspitler]

Now using astropy cookie cutter version, hopefully.

A few lingering issues. Will link relevant commits here:

• doc build seems to work, aside from the inheritance diagrams. Hope this is just a local install issue.
• why isn't Travis working?

Will submit PR now as draft until @AnthonyHorton tests thoroughly. Let's see if travis works...

Test using astropy's preferred automation helper via:
tox -e test

[lspitler]

Probably need to modify travis files to better match current version used by astropy, e.g.:
https://github.com/astropy/astropy/blob/master/.travis.yml
https://github.com/astropy/package-template/blob/cookiecutter/.travis.yml

[lspitler]

Alight the tests seem to pass without a need for changes in #39 . I created a fresh conda environment to test things and then ran:

cd gunagala_src_dir
git clean -xdf
pip install tox
tox -e test

Making this non-draft and ready for someone else to test and review.