White noise generators and AR covariance operators (#4716)

* White noise generators and AR covariance operators

---------

Co-authored-by: Jack Betteridge <j.betteridge@imperial.ac.uk>

Author: JHopeCollins

firedrake/adjoint/__init__.py

@@ -34,6 +34,10 @@
 )
 from firedrake.adjoint.ensemble_reduced_functional import EnsembleReducedFunctional  # noqa F401
 from firedrake.adjoint.transformed_functional import L2RieszMap, L2TransformedFunctional  # noqa: F401
+from firedrake.adjoint.covariance_operator import (  # noqa F401
+    WhiteNoiseGenerator, AutoregressiveCovariance,
+    PyOP2NoiseBackend, PetscNoiseBackend, VOMNoiseBackend,
+    CovarianceMat, CovariancePC)
 import numpy_adjoint  # noqa F401
 import firedrake.ufl_expr
 import types