Merge pull request #65 from astrofrog/fix-add-density-from-grid

Fix add density from grid
Initial C code to discretize 3-d Gaussian kernels
Work in progress: Python code to construct an Octree grid from SPH particles
Simplify octree construction
Fixed SPH importer, now functional but needs testing with real data.
Minor fix
Normalize mass in octree cells, and set density to zero in refined cells
More work on the SPH helpers