ENH: extend type aliases to include geometric boundary fields (#2348)

STYLE: LduInterfaceFieldPtrsList as alias instead of a class

STYLE: define patch lists typedefs when defining the base patch

- eg, polyPatchList typedef within polyPatch.H

INT: relocate GeometricField::Boundary -> GeometricBoundaryField

- was internal to GeometricField but moving it outside simplifies
  forward declarations etc. Code adapted from openfoam.org