To use a Krylov method inside the composite preconditioner, set the PCType of one or more
inner PCs to be PCKSP.
Using a Krylov method inside another Krylov method can be dangerous (you get divergence or
the incorrect answer) unless you use KSPFGMRES as the outer Krylov method
To use a different operator to construct one of the inner preconditioners first call PCCompositeGetPC(), then
call PCSetOperators() on that PC.