Pynbody

*Enhancements:*
* Add default mapping for black holes in GadgetHDF snapshots
* Better dependency specification in `setup.py` for automated installation (rokroskar)
* Handles Eagle-like GadgetHDF snapshots which store their halo/group catalogue as two arrays (specifying the group and subgroup number)
* Add ability to save back individual arrays in GadgetHDF format using `snap['arname'].write()` or `snap.family['arname'].write()` syntax
* Performance enhancements to GadgetHDF implementation

*Bug fixes:*
* Fixed an issue where mu (relative atomic mass) was defined incorrectly in some cases (mtremmel / trquinn)
* Fixed an issue in IndexedSubSnap, where an entire family array could be incorrectly returned instead of an empty array when no family members were present in the IndexedSubSnap
* Fixed an issue where inconsistent smoothing information would prevent a KDTree from generating density estimates; now KDTree insists on using its own smoothing lengths to prevent this occurring.

Bug fixes
- Fixed RGB star rendering on Python 3
- Fixed bugs affecting isolated runs with Gadget

Enhancements
- Better handling of how "eps" (softening) is passed to gravity routines to avoid dtype mismatches
- Introduced option to use a .units file to override the default inferred units of a file (Michele Mastropietro)
- Made improved error message for when a multi-threaded image rendering goes wrong on one or more threads

New features
- NFW profile fitting, with a framework for fitting other generic profiles (by Martin Rey)
- Halo shape fitting (by Matt Orkney)

* Reads Rockstar catalogues
* Refactors halo module into a subpackage for better readability and maintainability
* Improved GadgetHDF versatility

Adds support for new Ramses format, and fix bugs with HOP catalogue implementation

Bug-fix release addresses inappropriate default colour scaling in `pynbody.plot.sph.image` and related functions.