Fix MPIU_* routines to always return MPI error codes and thus be usable with PetscCallMPI()

Introduce MPIU_Count to be used when MPI_Count is not available

Add to CI compilers flags '-Wconversion', '-Wno-sign-conversion', '-Wno-float-conversion', '-Wno-implicit-float-conversion']

Also fix the code to repository to compile cleanly with these flags in th

Add to CI compilers flags '-Wconversion', '-Wno-sign-conversion', '-Wno-float-conversion', '-Wno-implicit-float-conversion']

Also fix the code to repository to compile cleanly with these flags in the CI

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PC: add new mat ops MATOP_GET_{BLOCK, VBLOCK}_DIAGONAL

so that a shell matrix can return its own (variable) point blocks in a matrix for use in PC (V)PBJacobi

Add shared memory support for PCMPI using both shmget() and MPI_Win_allocate_shared()

Mat: fix -Wmissing-field-initializers

This requires some changes to user FORTRAN code

Pass PETSC_NULL_XXX_ARRAY when passing a NULL as an argument that is expecting an array
Pass PETSC_NULL_ENUM when argument returns an enum instead of

This requires some changes to user FORTRAN code

Pass PETSC_NULL_XXX_ARRAY when passing a NULL as an argument that is expecting an array
Pass PETSC_NULL_ENUM when argument returns an enum instead of PETSC_NULL_INTEGER
Pass arrays (and not scalar values) when the argument is expecting an array; this means replace, for example, the argument v with [v]
Use PetscObjectIsNull(obj) to check if the object is NULL, instead of obj == PETSC_NULL_XXX

The compiler will now automatically prevent you from using the wrong argument type for the first three bullets above
This will also require an update sowing with the new sowing branch this MR is using.

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Remove now redundant increase in mat nonzero state

Fix serious bugs in MatSetValuesBlocked for SeqBAIJ and MPIBAIJ when a reallocation is needed.

Reported-by: Adrian Croucher <a.croucher@auckland.ac.nz>