[FEATURE] Support for Diffusion-weighted Imaging

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