ENH: Added tutorial case showing use of externalCoupled BCs

.../heatTransfer/buoyantSimpleFoam/externalCoupledCavity/0/T

.../heatTransfer/buoyantSimpleFoam/externalCoupledCavity/0/U

...Transfer/buoyantSimpleFoam/externalCoupledCavity/0/alphat

...ransfer/buoyantSimpleFoam/externalCoupledCavity/0/epsilon

.../heatTransfer/buoyantSimpleFoam/externalCoupledCavity/0/k

...eatTransfer/buoyantSimpleFoam/externalCoupledCavity/0/mut

...tTransfer/buoyantSimpleFoam/externalCoupledCavity/0/omega

.../heatTransfer/buoyantSimpleFoam/externalCoupledCavity/0/p

...tTransfer/buoyantSimpleFoam/externalCoupledCavity/0/p_rgh

...Transfer/buoyantSimpleFoam/externalCoupledCavity/Allclean

...atTransfer/buoyantSimpleFoam/externalCoupledCavity/Allrun

...r/buoyantSimpleFoam/externalCoupledCavity/Allrun-parallel

...ansfer/buoyantSimpleFoam/externalCoupledCavity/Allrun.pre

...atTransfer/buoyantSimpleFoam/externalCoupledCavity/README

...ntSimpleFoam/externalCoupledCavity/constant/RASProperties

...ansfer/buoyantSimpleFoam/externalCoupledCavity/constant/g

...oam/externalCoupledCavity/constant/polyMesh/blockMeshDict

...mpleFoam/externalCoupledCavity/constant/polyMesh/boundary

...m/externalCoupledCavity/constant/thermophysicalProperties

...er/buoyantSimpleFoam/externalCoupledCavity/externalSolver