Merge branch 'maint'
MatNestGetISs: use custom fortranstub to handle PETSC_NULL_OBJECTReported-by: "Klaij, Christiaan" <C.Klaij@marin.nl>
updated PETSc directory layout to match standard packaging strategiesinclude/petsc finclude,private,mpiunilib/petsc confbin/petsc*
manual merge of conflict
Change all PETSC_EXTERN_C to PETSC_EXTERNHg-commit: 8d2ebbb193fb583bccc64015e35640c4e08c3426
Custom binding for MatCreateNest() because it has nullable argsHg-commit: 5fbeef6d6e7232c0ce3be5e61ac2fa659f38f53f
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