Wrote 1D Voronoi cell generation algorithm and corresponding unit test. Algorithm is not coupled into swift yet.

Removed now-unnecessary large glass file from the repository. All examples download the required glass files from the web-server.

Fix the minimal version of SPH

Ok, I have now fixed the minimalistic implementation of SPH. That's the one that should be used as a template for other people to implement their stuff. It's overly documented and now works on all the test-cases we have in the examples directory.  

Don't know if you want to look at this or not @pdraper given that it does not affect our default setup at all.

Solves #148.

See merge request !218

First really working version of the Minimal SPH implementation. Need more documentation and testing.

First really working version of the Minimal SPH implementation. Need more documentation and testing.

Updated the 3D Sod shock example to use the new glass files. Added script to plot solution. Removed old glass files.

Fixed test125cells when using the Wendland kernels. Neighbour number tolerance was not large enough.

Broke one of my own rules by doing too much in a single commit: implemented runner_extra_ghost, moved gradient calculations for both gradient types to new gradient loop, fixed bug in engine.c involving the number of links (which is larger with an extra loop), streamlined gradient methods, documented most GIZMO specific files. Second loop works.