move PetscSFCreateFromLayouts() to PetscSF;

no need for #include <petscsf.h> in pmap.c

rename PetscLayoutsCreateSF -> PetscSFCreateFromLayouts

PetscSFSetGraphLayout: small optimization

Convert MPI error type to PETSc error with string message for all MPI calls

Now PETSc examples will ONLY return PETSc error codes and never MPI error codes directly so we can understand and post-pro

Convert MPI error type to PETSc error with string message for all MPI calls

Now PETSc examples will ONLY return PETSc error codes and never MPI error codes directly so we can understand and post-process their
errors better.

The test harness will now automatically retry tests that fail with MPI, this may help with Intel MPI that produces
seemingly random failures.

Commit-type: error-checking
/spend 30m

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SF: Place implementation of various utility routines in sf/utils

Turn on checkbadSource test to generate an error when found; fix all source code that causes errors

Commit-type: portability-fix, testing-fix, style-fix, feature, maintainability
/spend 1.5h

Vec: Replace 0 -> NULL for pointers