ENH: extend GeoMesh interface to include boundary_size() static method

- can be used to pre-allocate space for internal+boundary field for
  flat addressing (issue #3364).

  For example:

      DynamicField<Type> fld
      (
          std::pair<label,label>
          (
              GeoMesh::size(mesh),
              GeoMesh::size(mesh)
            + (extra ? GeoMesh::boundary_size(mesh) : label(0))
          )
      );

ENH: extend polyBoundaryMesh patch selectors

- indices_if() method with a predicate, or with a patch-type
  behaves similarly to findPatchIDs(), but returns a sorted
  labelList instead of a labelHashSet

- nFaces_if() method with a predicate return the number of boundary
  faces for patches matching the given predicate.