ENH: make PDRsetField ground, outer patch names configurable

- previously hard-coded, now adjustable within PDRsetFieldsDict

    // Change some predefined patch names
    patchNames
    {
        ground      ground;
        outer       outer;
    }

ENH: additions to PDRutils, improve comments

- expose enumerated expansion names and gridControl (PDRblock).
  Not commonly needed, but useful to have access when defining
  other grid generators

TUT: update PDRsetFieldsDict and tutorials to use "ground"

- remove tutorial references to unused types and legacy obstacles

- use "ground" for the boundary conditions instead of "seaGround".
  Consistent with PDRblockMesh