Decide on diffusion models

Implement standard model:

• sticks
• zeppelin
• tensor
• ball
• cylinder (restricted, time-dependent diffusion)
• sphere (restricted, time-dependent diffusion)
• dot

see https://www.sciencedirect.com/science/article/pii/S1053811911011566 as a good reference

All compartments assume the fibre lies along z. (still controversial) All compartments will build off compartment base class (example from Michiel).
Code for each compartment is stored in own file.
Implementing time dependent diffusion may be a secondary task, but the compartment code can still be written.

Inputs may be some combination of (amongst other things) bval, bvec, Delta (diffusion time). Output should be the diffusion signal along bvec.