dwidenoise: First working version of spherical kernel

New default behaviour is to use an expanding spherical kernel with number of voxels at least 1.1 times the number of volumes. For voxels near the edge of the image FoV, the radius of the kernel will increase until the requisite number of voxels is obtained.
Note that execution speed of this implementation seems to be reduced, even when using the cuboid kernel; this may be due to use of Eigen Blocks to denoise voxels with kernels smaller than the maximum processed.