ENH: Updated use of field sources in simple/pimple solvers

...solvers/incompressible/pimpleFoam/SRFPimpleFoam/UrelEqn.H

...ns/solvers/incompressible/pimpleFoam/SRFPimpleFoam/pEqn.H

applications/solvers/incompressible/pimpleFoam/UEqn.H

applications/solvers/incompressible/pimpleFoam/pEqn.H

...ns/solvers/incompressible/pimpleFoam/pimpleDyMFoam/UEqn.H

...ns/solvers/incompressible/pimpleFoam/pimpleDyMFoam/pEqn.H

...ns/solvers/incompressible/potentialFreeSurfaceFoam/UEqn.H

...ns/solvers/incompressible/potentialFreeSurfaceFoam/pEqn.H

...ns/solvers/incompressible/simpleFoam/MRFSimpleFoam/pEqn.H

...ns/solvers/incompressible/simpleFoam/SRFSimpleFoam/pEqn.H

applications/solvers/incompressible/simpleFoam/pEqn.H

applications/solvers/lagrangian/LTSReactingParcelFoam/UEqn.H

applications/solvers/lagrangian/LTSReactingParcelFoam/pEqn.H

applications/solvers/lagrangian/coalChemistryFoam/UEqn.H

applications/solvers/lagrangian/coalChemistryFoam/pEqn.H

.../lagrangian/porousExplicitSourceReactingParcelFoam/UEqn.H

.../lagrangian/porousExplicitSourceReactingParcelFoam/pEqn.H