Add tutorial case

tutorials/heatTransfer/chtMultiRegionSimpleFoam/heatExchanger: use fixedFluxPressure not buoyantPressure BCs

fixedFluxPressure BC: the snGrad is now pushed into the BC from pEqn.H rather than being evaluated in the BC

for laplacian of T in the solid for ppositeBurningPanels tutorial

    Time::adjustDeltaT(). It allows function objects to manipulate the time step to
    dump at adjustable times. The following options are available for output in
    function objects: timeStep, outputTime, adjustableTime, runTime, clockTime
    and cpuTime.