GAMG: switch the default to using square graph

Fix uneven back ticks

Fix typos

Remove first and last empty lines

add old aggressive coarsening algorithm as option, switch default MIS ordering algorithm back to old one

Fix typos

Convert all header guards to pragma once

Fix some malformed if !defined() header guards

Delete unused code

Add static to internal functions

Fix missing new lines at the end of PetscInfo() calls

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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... and apply it

Rename PetscValidPointer -> PetscAssertPointer

Deploy new PetscValidPointer, remove PetscValidXXXPointer