Change PETSc sys typedef function names to use the new _Fn format

Commit-type: housekeeping, maintainance

Fix places where PetscUseTypeMethod() and related macros should be used.

add typedef for functions passed to SNESSetFunction() etc to match approach in TS

I never liked Jed's use of typedef for function prototypes for a couple of reasons

1) you don't see immediately tha

add typedef for functions passed to SNESSetFunction() etc to match approach in TS

I never liked Jed's use of typedef for function prototypes for a couple of reasons

1) you don't see immediately that the argument to the function is a function pointer instead of standard variable when looking at source
2) you don't see at the exact location in the code the prototype of the function being passed in

but there is something to say for using these typedefs so

I've prototyped the change for SNES but slightly differently than what Jed did for TS

1) I append the typedef with _Ftn to mark that it is a function (helping to relieve the problem 1 above)
2) I made the typedef to be for a function, not a function pointer so it can also be used directly for things like

PETSC_EXTERN SNESJacobian_Ftn SNESComputeJacobianDefault;

I think the TS usage could be switched to this style without user code breakage, and deprecate the current TS typedef

Thoughts?

Just do SNES the same way as TS and live with less clarity?

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Merge remote-tracking branch 'origin/release'

SNESTestJacobian: fix for multilevel solvers

First batch of fixes

Fix the documented output type of SNESConvergedDefault.

LIBBASE is no longer used in make so remove it

Merge remote-tracking branch 'origin/release'

Merge branch 'stefanozampini/minsurf-local' into 'main'

Improve support for nonlinear domain decomposition solvers

See merge request petsc/petsc!7084

Update references in the manual pages to use Sphinx citation processing

Merge remote-tracking branch 'origin/release'

Fix -Wzero-as-null-pointer-constant errors

SNES: Add support to test finite differencing the objective function

SNESGetNPC: pass usercompute instead of application context if present

Merge remote-tracking branch 'origin/release'

SNESComputeObjective: add missing <rhs,x_k> term

Fix PetscOptionsGetViewer() so it works in a thread-safe way

When PetscOptionsGetViewer() returns, for example, PETSC_VIEWER_STDOUT_SELF the same object is shared
by all threads that call PetscOptio

Fix PetscOptionsGetViewer() so it works in a thread-safe way

When PetscOptionsGetViewer() returns, for example, PETSC_VIEWER_STDOUT_SELF the same object is shared
by all threads that call PetscOptionsGetViewer(). In the previous code PetscObjectReference() and
dereference (via PetscViewerDestroy() where called on the viewer by multiple threads after the viewer
was accessed thus corrupting the object.

This branch introduces a PetscOptionsRestoreViewer() that allows not doing the referencing and dereferencing
on persistant viewers, thus removing the specific race condition above.

Other possible solutions include

Use a lock on each PetscObjectReference/Dereference() just in case they are shared. Could be time consuming so one
could mark each object as either threadshared or independent and only do the locks on threadshared objects. This lowers
the cost but would require more extensive changes to PETSc infrastructure. And I do not know if this shared/not shared model
is what we need in the long run, though it might be.

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Merge remote-tracking branch 'origin/release'

Fix some manual pages focusing on SNES

Merge remote-tracking branch 'origin/release'

Docs: fix wrong occurences of then instead of than

Merge remote-tracking branch 'origin/release'