Firedrake backend: Spatial white noise sampling

[jrmaddison]

Uses the preconditioned form

$$X = \Xi^{-T} \sqrt{ \Xi^T M \Xi } Z,$$

where

• $Z$ has elements which are independent standard Gaussian.
• $M$ is the mass matrix.
• $\Xi$ is an approximation for the inverse of the mass matrix square root.
• The matrix square root is computed using SLEPc.

When preconditioning is enabled $\Xi$ is set equal to the (principal) square root of the diagonal of $M$.