do not import petsc4py.PETSc until we really need it.

[JHopeCollins]

When pyadjoint is imported we were checking if petsc4py.PETSc was available in the TAOSolver file whether or not the user ever actually used the TAOSolver.

The problem is that import petsc4py.PETSc will run petsc4py.init if it hasn't already been run, but it won't pass sys.argv.
This means that any PETSc options from the command line will silently be ignored. They won't even be shown by -options_left because PETSc never knew they even existed.

This PR delays importing petsc4py.PETSc and petsctools until they are actually needed so that the user has the chance to call petsc4py.init as they see fit.

[JHopeCollins]

@jrmaddison this doesn't change anything with the top level pyadjoint namespace, but it means that pyadjoint.optimization.tao_solver.PETSc and pyadjoint.optimization.tao_solver._reasons no longer exist so I just wanted to check if you were relying on either of them being there.

[jrmaddison]

Seems fine to move _tao_reasons, ideally might be moved to petsctools. A module level __getattr__ would also work for PETSc, petsctools, _tao_reasons, although as mentioned per function imports also seems fine.

[JHopeCollins]

Seems fine to move _tao_reasons, ideally might be moved to petsctools.

This is true, I forgot I had this old PR I didn't get around to finishing: firedrakeproject/petsctools#12