DRAFT: Add turtoisity

Abandoned for now, because the interplay of this with volume spin density, permeability, and bound states is very complicated.

src/geometries/internal/internal.jl

@@ -25,7 +25,7 @@ import .Obstructions:
     has_inside, radius, obstruction_type, size_scale, get_quadrant, random_surface_positions
 import .BoundingBoxes: BoundingBox, could_intersect, lower, upper
 import .RayGridIntersection: ray_grid_intersections
-import .Properties: R1, R2, off_resonance, permeability, surface_relaxivity, surface_density, dwell_time, max_timestep_sticking, MRIProperties
+import .Properties: R1, R2, off_resonance, diffusivity, permeability, surface_relaxivity, surface_density, dwell_time, max_timestep_sticking, MRIProperties
 import .Susceptibility: FixedSusceptibility, susceptibility_off_resonance, off_resonance_gradient
 import .IsInsideMesh: isinside_grid