DXT: include mpiio offset
The MPI-IO entries to the DXT file don't have any offset information. This pull request fixes that with calls to MPI_File_get_position and MPI_File_get_byte_offset.
These routines in ROMIO turn "counts of file types" into "bytes" by walking the flattened type description. In some really hairy cases (E3SM comes to mind) this could take a bit of time. ROMIO is going to go do the same type-walking internally so probably not adding a lot of overhead to an already challenging workload, but I wanted to point out that these routines are more costly than say 'fseek(fd, 0, SEEK_POS)'