Add D_inv struct to solver

[DanecLacey]

Some kernels need to divide by D, and it would be cheaper to multiply by D_inv. D_inv should be accessible everywhere that D is accessible.
@BallasticMonkey This is likely the reason for the slower Jacobi performance.

[DanecLacey]

D_inv is now used at least in the 2ST preconditioner. It's not supported throughout all methods, but small tests with Jacobi showed using D_inv vs. D didn't really make a difference. So it isn't a priority for now.