[petsc-dev] Error with ML preconditioner

[Mark F. Adams]

> 
> This is clearly wrong in the release too. I'm inclined to delete the entire useblockis code path since it seems like it will need to be implemented completely differently (if it matters for performance).

Yes it should be removed, I just deleted the whole section and tested it.

We could reinstate this code and check that all processors have a blocked index set before making blocked ISs.