PCMG matapply

This provides support for multivector application of PCMG. @prj- The KSPHPDDM tests in ksp ex26 fail. I'm using commit b442b45 for HPDDM. Can you please take a look? Note that the same tests are fine if run with KSPPREONLY. Feel free to add more hpddm related tests.

@prj- What is the purpose of having a composable KSP{Set|Get}MatSolveBlockSize? Why storing/retrieving the value stored in KSP (added in this MR) is not enough?