[FEATURE] Support for Diffusion-weighted Imaging

[LMZimmer]

Registration of Diffusion Weighted Images requires proper transformation of the diffusion tensors in the registration step. This likely requires implementation of tensor transformations in the registrator, as well as a DiffusionModality class and processing of the b vectors that come with DW images.

The following repository implements the necessary tensor transformations:

https://github.com/andeleyev/brain-tumor-tissue-reconstruction/blob/9112af31120e6774305507bf2fae1f0612b4015b/tissue_reconstruction/tissue_reconstruction.py#L88