First draft of adding perf-event attributes in hpcprof-mpi
First draft of adding perf-event attributes in hpcprof-mpi

Adding perf-event attributes in experiment.xml generated by hpcprof-mpi:
pep, pes and pem

This commit works fine for single hpcrun. For multiple hpcruns, the
attributes are taken from the first hpcrun. To be continued....
HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit,HPCToolkit/hpctoolkit