Initial attempt at computing individual block matrices simultaneously to speed up PBP (and others) when there are LOTS of variables.  Refs #3024 #3783
re-add the legacy computeJacobianBlock() function (although it calls the new function) for backwards compatibility and for simplicity when all you want to do is compute one block. refs #3024 #3783
Merge pull request #3866 from friedmud/block_preconditioning_3783

Initial attempt at computing individual block matrices simultaneously