Solve issues that occur when viewing angles are along walls

[astrofrog]

When one or more of the viewing angles are along a wall, or pass through the corner or edges of the cells, the following warnings are raised:

WARNING: not in correct cell - killing [grid_integrate]
WARNING: not in correct cell - killing [grid_integrate]
WARNING: not in correct cell - killing [grid_integrate]
WARNING: not in correct cell - killing [grid_integrate]
WARNING: not in correct cell - killing [grid_integrate]

and the photons get killed.

[astrofrog]

This is only really an issue for point sources, because it is the only source type where the starting position of a photon is exact, and may result in alignment with the grid walls, or cell vertices.

[astrofrog]

Actually, this could happen to any source provided that its dimensions are much smaller than the typical grid cell size.

[astrofrog]

In addition, even if the viewing angles are not aligned along walls, the current code will have difficulties with photons emitted from a point source that falls on a wall.

[astrofrog]

This has now been fixed for cartesian, spherical polar, and cylindrical polar grids.