ENH: use demand-driven schemes and solution w/o compatibility layer

applications/solvers/DNS/dnsFoam/createFields.H

applications/solvers/basic/potentialFoam/createControls.H

applications/solvers/basic/potentialFoam/createFields.H

...rs/basic/potentialFoam/overPotentialFoam/createControls.H

applications/solvers/combustion/PDRFoam/bEqn.H

applications/solvers/combustion/PDRFoam/createFields.H

applications/solvers/combustion/XiFoam/createFields.H

applications/solvers/combustion/XiFoam/ftEqn.H

applications/solvers/combustion/fireFoam/YEEqn.H

applications/solvers/combustion/fireFoam/createFields.H

applications/solvers/combustion/reactingFoam/YEqn.H

applications/solvers/combustion/reactingFoam/createFields.H

...ustion/reactingFoam/rhoReactingBuoyantFoam/createFields.H

...rs/combustion/reactingFoam/rhoReactingFoam/createFields.H

...ions/solvers/compressible/rhoCentralFoam/readFluxScheme.H

...olvers/compressible/rhoPimpleAdiabaticFoam/createFields.H

...cations/solvers/compressible/rhoPimpleFoam/createFields.H

...pressible/rhoPimpleFoam/overRhoPimpleDyMFoam/correctPhi.H

...essible/rhoPimpleFoam/overRhoPimpleDyMFoam/createFields.H

...cations/solvers/compressible/rhoSimpleFoam/createFields.H