Add Tukey Biweight and Huber’s minimax norm to medpy.filter.smoothing.anisotropic_diffusion

[cancan101]

Add a third and fourth option to medpy.filter.smoothing.anisotropic_diffusion. See Black, 1997 and this:

Biweight:

[loli]

Still thinking of implementing the fourth option of should I close this issue.

Btw.: Interestingly, in the formulation you cite here, the abs(s) is considered for c4, but not for c3. Maybe there is a reason behind this, although I can't think of any obvious.