GeometricField::GeometricBoundaryField -> GeometricField::Boundary

When the GeometricBoundaryField template class was originally written it
was a separate class in the Foam namespace rather than a sub-class of
GeometricField as it is now.  Without loss of clarity and simplifying
code which access the boundary field of GeometricFields it is better
that GeometricBoundaryField be renamed Boundary for consistency with the
new naming convention for the type of the dimensioned internal field:
Internal, see commit 4a57b9be

This is a very simple text substitution change which can be applied to
any code which compiles with the OpenFOAM-dev libraries.