Moose: petsc-3.5 API compliance: getting rid of MatStructure and Mat indirection in KSP/PCSetOperators() and friends.  Addresses #2348.
Merge pull request #2858 from karpeev/petsc-3.5

Moose: petsc-3.5 API compliance
Merge commit 'e35aa57db53d5935aa756fdc2d6f629b6261ca92'