New formulation for diffusion

ns * omegas = A * dns_dz + B * ns
New formulation for diffusion

ns * omegas = A * dns_dz + B * ns
Merge branch 'planet_grins' of https://github.com/SylvainPlessis/Planet_INC into planet_grins
Merge pull request #5 from SylvainPlessis/planet_grins

Planet grins