implement zeppelin compartment

The zeppelin compartment describes diffusion from an axially symmetric diffusion tensor, with parameters dax (axial diffusivity) and drad (radial diffusivity) i.e. diffusion parallel and perpendicular to the fibre.

The zeppelin can be expressed as
S = S0 exp(-b*dax*(bvec'*mu).^2)*exp(-b*drad(1-(bvec'*mu).^2)).

Inputs: S0, bval, bvec, axial_diffusivity, radial_diffusivity
Output: S (diffusion signal along bvec)

We will assume the fibre / tensor is oriented along the z axis (mu=[0;0;1]).