ENH: further tweak `Diffusion` gridding, return `tau_diffusion` from `Diffusion`
ENH: further tweak `Diffusion` gridding, return `tau_diffusion` from `Diffusion`

villrv/MOSFiT,villrv/MOSFiT,guillochon/MOSFiT,mnicholl/MOSFiT,bmockler/MOSFiT,mnicholl/MOSFiT,bmockler/MOSFiT,guillochon/MOSFiT,guillochon/FriendlyFit,guillochon/MOSFiT,mnicholl/MOSFiT