Generalize OneSidedStencil for fields other than SVol
As written, OneSidedOperatorTypes.h in spatialops/structured/stencil will most likely fail on staggered fields. The UnitType should be redefined to work correctly on other volume fields, such as XVol. It could also be generalized to include face fields. The test in spatialops/structured/stencil/test/test_one_sided_stencil.cpp should also be fixed to ensure that the test fields are staggered appropriately.