Unstructure the forward model code

[eigenvivek]

The 3D volume and the 2D detector plane are stored as their physical analogs (3D voxelgrid and 4D tensor of spatial coordinates, respectively). This is helpful for conceptualization, but there is no need to structure the code based on the geometry of the real world.

• The voxelgrid could be completely flattened, or hashed, so long as it's any structure with lookup.
• The spatial coordinates should be reshaped from (b, n1, n2, 3) where n1,n2 are image dimensions to (b, m, 3) where m=n1xn2. This would be especially useful for getting sparse computation of DRRs working (#22).