add another test for the interpolation
interp test: print values for specific point as well
add test_interp notebook
tiny bugfix for notebook
remove message about turblib bugfix
add pypi installation instructions
update local vs db interp, and relevant notebook

the point was to simplify the notebook a little bit.
add ipython notebook for testing divergence
divergence notebook: use every nskip-th y node
add notebook checkpoints to gitignore
minor bugfix for interp test

also, clean up output for interp test
use config folder for auth_token file
generate and use ~/.config/JHTDB folder

the authentication token is now a file in this folder.
the interpolator class also stores all of its different files in this
folder as well (polynomial data, c files, shared libraries).
the main reason for the folder is the interpolator class, since it takes
a while for these things to be generated.
add information about config folder in README
Merge branch 'config_folder' into develop
fix test.py for python3
make test_plain output prettier
Merge branch 'develop'