Dki

This is still WIP, but I think that I am about ready for others to also start taking a look at this. 

I am relying mostly on these two papers for the implementation details: 

    [1] Lu, H, Jensen, JH, Ramani, A, Helpern, JA (2006). Three-dimensional
    characterization of non-gaussian water diffusion in  humans using diffusion
    kurtosis imaging. NMR in Biomedicine 19: 236-247

    [2] Jensen, JH and Helpern JA (2010). MRI quantification of non-Gaussian
    water diffusion by kurtosis analysis. NMR in Biomedicine 23: 698-710.

However, I am trying to generalize this to cases in which measurements were conducted in different directions in the different b values. Not sure if that's legit.