YAXT

Depending on the special features of the global index space (map to physical space, incomplete subspaces) it would be nice to have elemental post-processing.

Optional features:

• Assign an undef value (given by the user) to points that could not be assigned during the transfer (instead of making this a fatal error). The user should also be able to query the number of local undef values. This is of general interest.

• Accept a sign change property of pairs of global index points - this allows us to correctly transform directed physical quantities if the map to physical space includes point symmetries. This is really special but has one important use case: MPIOM.