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.

Wrote dimension dependent matrix inversion method and corresponding unit test. Started making Gizmo dimension independent.