Asynchronous I/O requirements

These are suggested features for the asynchronous I/O support in the library.

@ambrad :

- Summit: 6 resource sets (RS) per node. 1 rank per RS drives the 1 GPU/RS. a second rank per RS drives I/O for the RS.
- can choose async/sync per file. e.g., do it only for restart files.
- if MPI comm b/w for I/O can be large, provide an option to throttle it. e.g., a mid-run restart file could take several hours to write asynchronously and with throttled comm. perhaps there would have to be a throttle-override call that would tell PIO to run at max speed at the end of a run.
- all operations in addition to writing that can be async should support it: e.g., openfile, initdecomp, closefile
- related: support ADIOS vs PNETCDF per file.