Set local matrix diagonal without parallel communication

[garth-wells]

Since #1417 we need to assemble (parallel comm) the matrix after regular assembly and before setting a local diagonal, set diagonal, and the assemble (parallel comm) again. Previously only one parallel comm step was required.

We should figure out how to set the local diagonal without requiring a parallel communication step afterwards.

[chrisrichardson]

Is this a general requirement, or a quirk of PETSc?

[garth-wells]

Is this a general requirement, or a quirk of PETSc?

PETSc.