GeometricField: New non-const access function boundaryFieldRef()

There is a need to specify const or non-const access to a non-const
object which is not currently possible with the "boundaryField()" access
function the const-ness of the return of which is defined by the
const-ness of the object for which it is called.  For consistency with
the latest "tmp" storage class in which non-const access is obtained
with the "ref()" function it is proposed to replace the non-const form
of "boundaryField()" with "boundaryFieldRef()".

Thanks to Mattijs Janssens for starting the process of migration to
"boundaryFieldRef()" and providing a patch for the OpenFOAM and
finiteVolume libraries.