feature request: darshan-parser to add a field of MPI ranks that do file I/O

[wkliao]

In MPI I/O, cb_nodes hint indicates the number of ranks that do actually file I/O
(e.g. POSIX calls) in a collective call. However, this hint may be ignored by the
underlying MPI-IO implementation and MPI-IO may not report this hint as the
real value that is used.

Because Darshan's POSIX heat maps generated by darshan-parser display all
the rank IDs that have non-zero sized activities, it should not be too difficult to
collect the information about

• number of MPI ranks that do POSIX I/O, and
• their rank IDs