#10 add option to use iterational random walk for smoothing on DelayedArrays

The possible usage of the DelayedArray data structure and the smoothing in chunks provides the possibility to reduce the memory requirements of the package. However, the calculation of the diffusion/smoothing kernel K is itself a memory intensive operation. The implementation of an alternative iterative smoothing procedure (like in the randomWalkByIterations function) on the chunks and an additional input parameter for the user to choose this approach would provide another option to reduce the memory usage even more.