Merge pull request #61 from astrofrog/allow-no-stokes-images

Implement a way to disable Stokes components in images
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
Added test for SPH gridding
Ensure tests for importers get installed
Added missing __init__.py for importers.tests
Added reference data file
Comment out unused parallel code for SPH gridding
Added function for stopping criterion, and write documentation
Added changelog entry
Added docs about writing/reading octree grids