Fix indenting in siena_diff.cc

It is with great reluctance that I push this PR on the lowest level of pedantry, indenting. However, the key C++ code of the widely used tool siena, siena_diff.cc, had wildly inconsistent indenting (i.e. a massive core voxel-wise loop that starts out logically indenting, and then wildly fluctuates in indenting) that made it nearly impossible to read.

Inferring that siena_diff.cc was originally written with a 2-space tradition, I have kept to this, and as a final step converted all tabs to spaces to avoid future corruption of the indenting.

Finally, this was to be an all-white-space change PR, but instead I added exactly 3 non-white-space changes, comments to clarify the closings of the large inner loops.