implement sphere compartment

This compartment models restricted gaussian diffusion in a sphere with a non-zero radius. This model is diffusion time-dependent, so requires more specific acquisition parameters including DELTA (diffusion time), delta(gradient time), and G (gradient strength).

The model parameters are radius, diffusivity, and signal at b0 (S0). For a further reading look at this paper:

G.J. Stanisz, A. Szafer, G.A. Wright, R.M. Henkelman An analytical model of restricted diffusion in bovine optic nerve