unify values used for cell/vertex centering in AMReX and CarpetX

[rhaas80]

Currently AMReX's CELL and VERTEX enums are 0 and 1 while CarpetX using 1 and 0 for cell and vertex centered data (user visible in loop<IDXTYPE, IDYTYPE, IDZTYPE> templates).

To avoid confusion this should be made uniform among the library and its client code.