MatCreateSubMatrices: rectangular blocks with SBAIJ

Minor documentation fixes

Important fixes

Add static to internal functions

Add convenience functions VecCreateFromOptions() and MatCreateFromOptions() to reduce the use of specialize creation routines in the examples and thus make it easier for people to use GPU based solve

Add convenience functions VecCreateFromOptions() and MatCreateFromOptions() to reduce the use of specialize creation routines in the examples and thus make it easier for people to use GPU based solvers

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Profiling: Improve !defined(PETSC_USE_LOG) #defines

- If we cast macro arguments to void, we can avoid unused errors that have
forced lots of conditionally included code like this:

```c
#if def

Profiling: Improve !defined(PETSC_USE_LOG) #defines

- If we cast macro arguments to void, we can avoid unused errors that have
forced lots of conditionally included code like this:

```c
#if defined(PETSC_USE_LOG)
PetscLogEvent event;
#endif
```

- Having global counters like petsc_TotalFlops externed event
if !PetscDefined(USE_LOG) helps to remove a lot of needless
`#ifdef`s around code that can instead be placed in
`if (PetscDefined(USE_LOG)) {}` blocks.

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Fix the places that should be using PETSC_STATIC_ARRAY_LENGTH

... and apply it

Rename MatChop() into MatFilter()

Rename PetscValidPointer -> PetscAssertPointer

Deploy new PetscValidPointer, remove PetscValidXXXPointer

MatEliminateZeros(): skip zero nondiagonal coefficients

Expose MatComputeVariableBlockEnvelope() in public headers

Manual linter fixes: mat

Merge branch 'jolivet/fix-Wextra-semi-stmt' into 'main'

Fix -Wextra-semi-stmt

See merge request petsc/petsc!6708

Fix some typos and documentation mismatches

Fix -Wextra-semi-stmt

Lint apply: mat