Sources should return a non-null VoxelDimensions

[bogovicj]

AbstractSources, and therefore RandomAccessibleIntervalSource return a null VoxelDimensions.

I think I'd prefer returning a "default" with spacings of "1" with units of "pixels" or "voxels". Without this, anything depending on a Source that uses VoxelDimensions have to check whether it's null or not (e.g. bigwarp - see this gitter conversation)

Any objections?

@hanslovsky
@kephale
@tpietzsch
@axtimwalde

[axtimwalde]

Agree.

[hanslovsky]

I am OK with a returning a reasonable non-null default VoxelDimension. It does not affect me either way because I ignore the VoxelDimensions in all my projects, for two reasons:

1. In Paintera, "global" space is in nano meters (implicit).
2. The meaning of VoxelDimensions is not entirely clear to me. Is it the size of a voxel of the transformed source in global space? Or is it the size of the transformed source before the transformation is applied? Does the source transform encode the VoxelDimensions anyway?

[bogovicj]

Well, the above has the easiest reasonable, non-breaking fix.

With the complication that, to prevent this from changing the constructor of AbstractSource,
the default voxeldimensions itself can't know how many dimensions it has, so this looks ugly.

Should have written in the comment that the number of dimensions there does not matter, since
DefaultVoxelDimensions will always return 1 as its spacing..

If we're okay with this, I can PR (after giving a documenting the new DefaultVoxelDimensions class.

[tpietzsch]

sounds good

[bogovicj]

PR #43 merged. Thanks!