iterative ref: more tests for insert_slice

[geoffwoollard]

Ideas for tests

• Using random rotation.
• check sum is close, no matter the inserted values.
• insert zeros should get zeros
• insert ones along axis (xy,xz,yz plane) should get ones
• most of entries should be zero (ratio of box length, maybe up to a factor of 2 or 3)
• same filled voxels between counts and inserted_slice_3d