Clean up 32 constants

[philippotto]

We have some magic numbers in the code base, but the problematic one is 32. In several places, it's hardcoded which is confusing and often times problematic, since the actual wkw header can contain different values here.

Also, one should carefully distinguish between the file_len and the block_len (how many blocks are in one cube? and how many voxels are in a block?). This is the naming wkw uses, but we can also use another terminology. However, it should be consistent in the code base.