gets the inner
PetscSF implementing gathers and scatters
sf - star forest that may contain roots with 0 or with more than 1 vertex
multi - star forest with split roots, such that each root has degree exactly 1
In most cases, users should use
PetscSFScatterBegin() instead of manipulating multi
directly. Since multi satisfies the stronger condition that each entry in the global space has exactly one incoming
edge, it is a candidate for future optimization that might involve its removal.