Add biased stellar emission for external sources

[karllark]

It would be more efficient to have biased emission in the case of a source external to the dust grid. This is clear in the case for the Slab benchmark where many of the photons emitted isotropically from the star do not intersect with the slab. It would be better to be able to specify a emission direction and solid angle (via circular spot with angular radius). We may still have some photons that do not intersect the dust grid, but it would be many fewer.