Ldtk

Final cleanup and bugfixes

- Fixed a Python 3 compatibility issue in saving and loading LDPSets
- Added a set of basic transmission curves (sloan g, r, i, and z passbands approximated with Boxcar filters, and Kepler response function) that can be imported from `ldtk.filters`
- Upgraded the basic example notebook
- Changed to use setuptools in `setup.py`

- Python 3 compatibility fixes.

Bugfixes and a square root limb darkening model provided by Rainer Wichmann.
- Bugfix: The TEFF grid for TEFF > 7000 K was faulty, now fixed.
- Bugfix: The second term in the nonlinear limb darkening model had a wrong coefficient.
- Feature: Square root limb darkening model.

Python 2 & 3 compatibility.

- Fixed a bug that broke the renormalization of z an mu values (and resampling)
- Attempted to make the package MPI friendly (now only the root instance attempts to download uncached files)
- Changed to use tqdm progress bar
- Now allowing for an instance-specific LDTk cache directory to make sharing projects easier
- Minor cleanup here and there