Merge pull request #18 from low-sky/master

Cleanup and testing of AKL changes.
Allow astropy kernels for beams and small fixes.
Calculate scale with positive values.
Using std returns single value. Choose scale method at __init__.
Pass additional args to spatial_smooth.
Change scale_cube and noise_cube to properties to avoid recomputing.
Removed recomputing of scale in spatial_smooth. compute_scale reverts to STD if there are no negative values.
Revert to getting flattened array.
Merge pull request #20 from e-koch/master

Okay; looks good.